[ruby] drop ruby 2.6 support (#34198)

As title

Rakefile

@@ -144,7 +144,7 @@ task 'gem:native', [:plat] do |t, args|
   verbose = ENV['V'] || '0'
 
   grpc_config = ENV['GRPC_CONFIG'] || 'opt'
-  ruby_cc_versions = ['3.2.0', '3.1.0', '3.0.0', '2.7.0', '2.6.0'].join(':')
+  ruby_cc_versions = ['3.2.0', '3.1.0', '3.0.0', '2.7.0'].join(':')
   selected_plat = "#{args[:plat]}"
 
   # use env variable to set artifact build paralellism

tools/bazelify_tests/dockerimage_current_versions.bzl

@@ -58,7 +58,6 @@ DOCKERIMAGE_CURRENT_VERSIONS = {
     "tools/dockerfile/distribtest/python_ubuntu2004_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/python_ubuntu2004_x64@sha256:342e9dc23b674ad256b220745745be818708a1baa25a2690f0d4f777e28a22a3",
     "tools/dockerfile/distribtest/ruby_centos7_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_centos7_x64@sha256:4d529b984b78ca179086f7f9b416605e2d9a96ca0a28a71f4421bb5ffdc18f96",
     "tools/dockerfile/distribtest/ruby_debian10_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_debian10_x64@sha256:1298c39c950b2a48261555b6cff1ae66230a5020f100d3b381759285f0caf84e",
-    "tools/dockerfile/distribtest/ruby_debian10_x64_ruby_2_6.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_debian10_x64_ruby_2_6@sha256:3ef9a84a77276f2ccaca3c97336573f76ed02719645e4bb67ccdf1f33da99fc8",
     "tools/dockerfile/distribtest/ruby_debian10_x64_ruby_2_7.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_debian10_x64_ruby_2_7@sha256:5ee26ad3abe2683c9a8ee03987ab0ae63f50793c3d3f5e4be6e6cbacb4556fcf",
     "tools/dockerfile/distribtest/ruby_debian10_x64_ruby_3_0.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_debian10_x64_ruby_3_0@sha256:9190da90a2a95eca1370cef64dcba7ddee9f59cc7487093da6711c1280a0b0f9",
     "tools/dockerfile/distribtest/ruby_ubuntu1604_x64.current_version": "docker://us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_ubuntu1604_x64@sha256:e0276968184a6c1e16de4e6afbbd469df91b27e40d061340841c76e864fdcb50",

tools/dockerfile/distribtest/ruby_debian10_x64_ruby_2_6.current_version

@@ -1 +0,0 @@
-us-docker.pkg.dev/grpc-testing/testing-images-public/ruby_debian10_x64_ruby_2_6:49b4854d213d0219ae41d4cfe38192df26958aa8@sha256:3ef9a84a77276f2ccaca3c97336573f76ed02719645e4bb67ccdf1f33da99fc8

tools/dockerfile/distribtest/ruby_debian10_x64_ruby_2_6/Dockerfile

@@ -1,42 +0,0 @@
-# Copyright 2015 gRPC authors.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-FROM debian:10
-
-# Install Git and basic packages.
-RUN apt-get update && apt-get install -y \
-  gnupg2 \
-  procps \
-  curl \
-  gcc && apt-get clean
-
-#==================
-# Ruby dependencies
-
-# Install rvm
-RUN gpg2 --keyserver hkp://keyserver.ubuntu.com --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 7D2BAF1CF37B13E2069D6956105BD0E739499BDB
-RUN \curl -sSL https://get.rvm.io | bash -s stable
-
-# Install Ruby 2.6
-RUN /bin/bash -l -c "rvm install ruby-2.6.5"
-RUN /bin/bash -l -c "rvm use --default ruby-2.6.5"
-RUN /bin/bash -l -c "echo 'gem: --no-document' > ~/.gemrc"
-RUN /bin/bash -l -c "echo 'export PATH=/usr/local/rvm/bin:$PATH' >> ~/.bashrc"
-RUN /bin/bash -l -c "echo 'rvm --default use ruby-2.6.5' >> ~/.bashrc"
-RUN /bin/bash -l -c "gem install bundler --no-document"
-
-RUN mkdir /var/local/jenkins
-
-# Define the default command.
-CMD ["bash"]