RELEASE-NOTES-1.31: Add in other cherry-picks since 1.31.1 was cut

RELEASE-NOTES-1.31: Add in other cherry-picks since 1.31.1 was cut

Gone through `git log --topo-order --no-merges --reverse 1.31.1..`
from 1f664ea4 to 7a6393fc (HEAD as of writing); re-worded a couple,
grouped the PHP version work together, and skipped a couple which
were just follow-up tweaks or test fixes to ones already in the list.

Change-Id: Ic04998209348abf73eefb1cad404700da91457ed