Our Mac test runs recently started failing to download RVM. The issue
appears to be a combination of an SSL certificate expiring and old
OpenSSL versions having a bug preventing them from validating the
replacement certificate: rvm/rvm#5133

This commit works around the problem by downloading RVM from GitHub as
suggested in one of the comments on the issue above.

The benchmark runs have been failing since we started requiring Python
3, so this changes fixes the benchmarks by ensuring we always use Python
3.

https://www.python.org/downloads/release/python-3100/

* 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]>

Merge 3.18.x into 3.19.x

Our Ruby release build broke at some point in the past day or so, and I
strongly suspect it is due to a new version (1.2.0) of
rake-compiler-dock. This commit pins the version to 1.1.0 as a temporary
fix.

Credit should go to @elharo for most of these Java changes--I am just
cherry-picking them from our internal codebase. The one thing I did
change was to give the UTF-8 validation tests their own Bazel test
target. This makes it possible to give the other tests a shorter
timeout, which is important for UnknownFieldSetPerformanceTest in
particular.