Initial set of Gerrit v2.16.23 release notes

Start the Gerrit v2.16.23 release notes with an initial extraction
of the major fixes and improvements introduced since the last
version.

Also adjust the headers by removing, for better consistency, the
trailing ':' at the end of each chapter.

Change-Id: I7ec134f62babbac6716988e6b0870689d53c7856
1 file changed