Starting the Emacs 27 release cycle

[Eli Zaretskii]

I've created the emacs-27 release branch, and bumped Emacs version on
master to 28.0.50.  A pretest of Emacs 27.1 will follow some time
soon, I hope.

Please from now on push safe bugfixes and documentation fixes to the
release branch; all the other development should happen on master.
When in doubt whether a bugfix is safe enough, please ask.

CONTRIBUTE has more about working with master and the release branch,
please review those practices.

Please also checkout and build the release branch and use the version
there to find potential bugs sooner rather than later.

TIA