get-pick-list: Ignore commits which CC mesa-stable unless they say "9.1"

With the recent creation of the 9.2 branch, we're now seeing a steady flow of
new patches to mesa-stable@ intended for that branch. To avoid wasting effort
trying to apply these to 9.1, we now require developers to specifically
mention "9.1" to target the 9.1 branch.
Bump version to 9.1.6
docs: Add release notes for 9.1.6 release
docs: Add 9.1.6 release md5sums
docs: Fix a typo in the 9.1.6 release notes
mesa: Fix MESA_PATCH version