[Test] Changed gcc-7 to gcc-8 (#34444)

Bumping gcc 7 to 8 to workaround the ongoing gcc segfault problem when
building Protobuf C++. Currently Foundational C++ requires gcc 7 so this
is a temporary measure to make the test green. We need to either make a
decision to change the minimum version of gcc in the Foundational C++ or
find a way to support gcc 7 without gcc segfault soon.

templates/tools/dockerfile/test/cxx_gcc_8_x64/Dockerfile.template

@@ -14,7 +14,7 @@
   # See the License for the specific language governing permissions and
   # limitations under the License.
   
-  FROM gcc:7
+  FROM gcc:8
   
   RUN apt-get update && apt-get install -y curl git time wget zip && apt-get clean
   <%include file="../../git_avoid_dubious_ownership_error.include"/>

tools/bazelify_tests/dockerimage_current_versions.bzl

@@ -102,7 +102,7 @@ DOCKERIMAGE_CURRENT_VERSIONS = {
     "tools/dockerfile/test/cxx_debian11_x86.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/cxx_debian11_x86@sha256:cba4b92ff05bc51c0668bfce696d4a826728f4c61163b6d6107685cd83098b15",
     "tools/dockerfile/test/cxx_debian12_openssl309_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/cxx_debian12_openssl309_x64@sha256:a3f1242283eb5988e4aefede8af64db6f4bfea0d2875610cf55560abeb60563e",
     "tools/dockerfile/test/cxx_gcc_12_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/cxx_gcc_12_x64@sha256:bbdfe66f27b964f9bfd526646b94a19d904fea52bdb244f32fd4355cc8c4551f",
-    "tools/dockerfile/test/cxx_gcc_7_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/cxx_gcc_7_x64@sha256:fb1924844078f48557d6ab57eac1482f80a3cc216406efc3aeaecab671c886d5",
+    "tools/dockerfile/test/cxx_gcc_8_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/cxx_gcc_8_x64@sha256:a12780fc969d6856211bb80d85de22198bcf0b2f325d1f2e5cc2d779b41ee919",
     "tools/dockerfile/test/php73_zts_debian11_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/php73_zts_debian11_x64@sha256:bc221d435086d92b2482020214ee70814c569273f8d0cb577a0247e82d598a5f",
     "tools/dockerfile/test/php7_debian11_arm64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/php7_debian11_arm64@sha256:7ee21f253a2ddd255f4f6779cd19818eec6524e78b0bf0a7765339e4aa7347c3",
     "tools/dockerfile/test/php7_debian11_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/php7_debian11_x64@sha256:302c06c5dbffb97dd5540d758a8ce849269527bb7d1c3885af0b956f8f33c49e",

tools/dockerfile/test/cxx_gcc_7_x64.current_version

@@ -1 +0,0 @@
-us-docker.pkg.dev/grpc-testing/testing-images-public/cxx_gcc_7_x64:8bc7b7f679857e741188d07a7db7acecf3a9168f@sha256:fb1924844078f48557d6ab57eac1482f80a3cc216406efc3aeaecab671c886d5

tools/dockerfile/test/cxx_gcc_8_x64.current_version

@@ -0,0 +1 @@
+us-docker.pkg.dev/grpc-testing/testing-images-public/cxx_gcc_8_x64:b28a69d75b25581e20dafd4356bca3e0d26e3d0e@sha256:a12780fc969d6856211bb80d85de22198bcf0b2f325d1f2e5cc2d779b41ee919

tools/dockerfile/test/cxx_gcc_8_x64/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM gcc:7
+FROM gcc:8
 
 RUN apt-get update && apt-get install -y curl git time wget zip && apt-get clean
 #=================