mesa: Bump version to 9.2.1

In preparation for the 9.2.1 release, of course.
docs: Add release notes for 9.2.1 release

With the list of bugs fixed and a full list of changes.
docs: Add md5sums for 9.2.1 release

Which we could only do after creating the tar files, of course.
docs: add missing <pre> tag