Version bump for a release (apple#1852)

Prepare for a 1.31.0 release and update docs (apple#1843)

# Motivations

In the previous PRs, I landed support for distributing `protoc` via this
repo which finally allows us to broadly recommend the SPM plugin.

# Modifications

This PR bumps the versions and updates the docs for the SPM plugin and a
tiny addition to the RELEASING doc to ensure the latest check is set.

# Result

Unblocking the ecosystem in broadly adopting the SPM plugin.

Fix `make_latest` from Boolean to String (apple#1838)

Version bump to prep for a release (apple#1781)

Version bump for a release (apple#1757)

Version bump for a release.

Version bump for a release.

Version bump for a release.

Version bump for a release.

Move `main` to 1.27.0

The next release will be 1.27.0 and will come off `main`.