Cherry-pick fix from #9311 into 3.18.x (#9320)

* Dropped Ruby 2.4 support for CI and releases.

* Removed Kokoro configs for old Ruby versions.

* Update Dockerfile to allow it to be rebuilt

Co-authored-by: Joshua Haberman <[email protected]>

Author: Adam Cozzette

kokoro/linux/dockerfile/test/ruby/Dockerfile

@@ -23,13 +23,11 @@ RUN apt-get update && apt-get install -y \
   && apt-get clean
 
 # Install rvm
-RUN gpg --keyserver hkp://keys.gnupg.net --recv-keys \
+RUN gpg --keyserver hkp://keyserver.ubuntu.com --recv-keys \
     409B6B1796C275462A1703113804BB82D39DC0E3 \
     7D2BAF1CF37B13E2069D6956105BD0E739499BDB
-RUN \curl -sSL https://get.rvm.io | bash -s master
+RUN \curl -sSL https://raw.githubusercontent.com/rvm/rvm/master/binscripts/rvm-installer | bash -s master
 
-RUN /bin/bash -l -c "rvm install 2.3.8"
-RUN /bin/bash -l -c "rvm install 2.4.5"
 RUN /bin/bash -l -c "rvm install 2.5.1"
 RUN /bin/bash -l -c "rvm install 2.6.0"
 RUN /bin/bash -l -c "rvm install 2.7.0"