The legacy thread annotations were spellings that were not prefixed
with ABSL_ and caused conflicts with other libraries. These spellings
have been removed by default for a while, but could have been added
back with the temporary compile flag
-DABSL_LEGACY_THREAD_ANNOTATIONS. This change completely removes the
legacy spellings.

If you were relying on the legacy spellings, the fix is
straightforward. Simply add the ABSL_ prefix. For example,
GUARDED_BY() becomes ABSL_GUARDED_BY().

PiperOrigin-RevId: 560874026
Change-Id: Id072e67435472220ea4f43ccbf267028c13feba7