[Test] RBE back to Ubuntu 20.04 (#34720)

Partial rollback of https://github.com/grpc/grpc/pull/34473 for RBE as
RBE needs Ubuntu 20.04 to get clang artifacts which Google publishes.
There aren't artifacts for Ubuntu 22.04 yet so there is no point of
upgrading RBE image to 22.04.

templates/tools/dockerfile/test/rbe_ubuntu2004/Dockerfile.template

@@ -14,9 +14,9 @@
   # See the License for the specific language governing permissions and
   # limitations under the License.
   
-  FROM ubuntu:22.04
+  FROM ubuntu:20.04
 
-  # Workaround the "tzdata hangs apt-get install during docker image build" issue for ubuntu22.04
+  # Workaround the "tzdata hangs apt-get install during docker image build" issue for ubuntu20.04
   # by setting timezone to "Etc/UTC" in advance.
   RUN echo "Etc/UTC" > /etc/timezone
 
@@ -35,7 +35,7 @@
   # * for sanitizers to work, we need MSAN and TSAN enabled versions of libc++
   # Check https://storage.googleapis.com/clang-builds-stable/clang-ubuntu20_04/latest.txt
   # for the value of LLVM_LATEST.
-  ARG LLVM_LATEST="f3d0613d852a90563a1e8704930a6e79368f106a"
+  ARG LLVM_LATEST="f2b94bd7eaa83d853dc7568fac87b1f8bf4ddec6"
   RUN wget -q https://storage.googleapis.com/clang-builds-stable/clang-ubuntu20_04/clang_r<%text>${LLVM_LATEST}</%text>.tar.gz -O /tmp/clang.tar.gz ${'\\'}
     && tar -xzf /tmp/clang.tar.gz -C /usr/local && rm /tmp/clang.tar.gz
   RUN wget -q https://storage.googleapis.com/clang-builds-stable/clang-ubuntu20_04/libcxx_r<%text>${LLVM_LATEST}</%text>.tar.gz -O /tmp/libcxx.tar.gz ${'\\'}

tools/bazelify_tests/dockerimage_current_versions.bzl

@@ -109,7 +109,7 @@ DOCKERIMAGE_CURRENT_VERSIONS = {
     "tools/dockerfile/test/python_alpine_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/python_alpine_x64@sha256:75fa424f2dae683422a5875d64911d9abf06c31e944401d240666d06f83de573",
     "tools/dockerfile/test/python_debian11_default_arm64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/python_debian11_default_arm64@sha256:e1fd0a85aed633d817ac086f3c588c6343d316b7d41a4dd62a8ecfbb70358687",
     "tools/dockerfile/test/python_debian11_default_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/python_debian11_default_x64@sha256:8f22ba10b93b14c13ceb296cbbd41dba04947b457efc229412b43280ad6a4adc",
-    "tools/dockerfile/test/rbe_ubuntu2204.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/rbe_ubuntu2204@sha256:a193b5d481ddd24cffbbe31d81c95f97e5a9f417fdd6ba8b7be972c28dfaa9e7",
+    "tools/dockerfile/test/rbe_ubuntu2004.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/rbe_ubuntu2004@sha256:d3951aeadf43e3bee6adc5b86d26cdaf0b9d1b5baf790d7b2530d1c197adc9f8",
     "tools/dockerfile/test/ruby_debian11_arm64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_debian11_arm64@sha256:7e77cf17e2e8657f4cc23ac9f93630bf13213fff961799e0f16dae17cd45cf6d",
     "tools/dockerfile/test/ruby_debian11_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_debian11_x64@sha256:e4cb502caccf2db733268ce2ddc951fda8a9df2f7f53d6b74523c33d40c83006",
     "tools/dockerfile/test/sanity.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/sanity@sha256:49745399fdef994b033fd8a5cad293d822ac25289770b7c342e34e781ff41590",

tools/dockerfile/test/rbe_ubuntu2004.current_version

@@ -0,0 +1 @@
+us-docker.pkg.dev/grpc-testing/testing-images-public/rbe_ubuntu2004:58594aefea3481a4839f8411e365b58ef9f34fcc@sha256:d3951aeadf43e3bee6adc5b86d26cdaf0b9d1b5baf790d7b2530d1c197adc9f8

tools/dockerfile/test/rbe_ubuntu2004/Dockerfile

@@ -12,9 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM ubuntu:22.04
+FROM ubuntu:20.04
 
-# Workaround the "tzdata hangs apt-get install during docker image build" issue for ubuntu22.04
+# Workaround the "tzdata hangs apt-get install during docker image build" issue for ubuntu20.04
 # by setting timezone to "Etc/UTC" in advance.
 RUN echo "Etc/UTC" > /etc/timezone
 
@@ -57,7 +57,7 @@ RUN ln -s /usr/bin/python3 /usr/bin/python
 # * for sanitizers to work, we need MSAN and TSAN enabled versions of libc++
 # Check https://storage.googleapis.com/clang-builds-stable/clang-ubuntu20_04/latest.txt
 # for the value of LLVM_LATEST.
-ARG LLVM_LATEST="f3d0613d852a90563a1e8704930a6e79368f106a"
+ARG LLVM_LATEST="f2b94bd7eaa83d853dc7568fac87b1f8bf4ddec6"
 RUN wget -q https://storage.googleapis.com/clang-builds-stable/clang-ubuntu20_04/clang_r${LLVM_LATEST}.tar.gz -O /tmp/clang.tar.gz \
   && tar -xzf /tmp/clang.tar.gz -C /usr/local && rm /tmp/clang.tar.gz
 RUN wget -q https://storage.googleapis.com/clang-builds-stable/clang-ubuntu20_04/libcxx_r${LLVM_LATEST}.tar.gz -O /tmp/libcxx.tar.gz \

tools/dockerfile/test/rbe_ubuntu2204.current_version

@@ -1 +0,0 @@
-us-docker.pkg.dev/grpc-testing/testing-images-public/rbe_ubuntu2204:53c2d4dd8ec1799158a3a0fc5eb6fc78906e9bac@sha256:a193b5d481ddd24cffbbe31d81c95f97e5a9f417fdd6ba8b7be972c28dfaa9e7