Update 2.8 release notes with changes from stable-2.6 and stable-2.7

The stable-2.6 and stable-2.7 branches have been removed, and there
will be no further releases on the 2.6 and 2.7 series.

All the changes merged into those branches since the 2.7 release are
already released in 2.8.

Add the changes in the release notes.

Change-Id: I191846f08c1545dadae28eb167a8fc4195e0dbdb
1 file changed