Updated version to 4.0.0-rc2

Updated CHANGES.txt to reference the new version 4.0.0-rc1.

Update protobuf version

Merge pull request protocolbuffers#7590 from haberman/merge-release-b…

…ranch

Merge release branch

Updated the changelog for the 3.12.2 release.

Added changelog entry for 3.12.1.

Added a changelog entry about the Java fix. (protocolbuffers#7516)

Updated Changelog for 3.12.0-rc2.

Added changelog entry for removing binary gems for Ruby <2.5. (protoc…

…olbuffers#7454)

Skip building distribution tar files for 32 bit Mac (protocolbuffers#…

…7214)