Strategy:

Merge latest master into spm-2926-gtest branch as usual.

Make branch spm-2926-gtest-staging-base
master as base, then:
1 commit of vendoring gtest
1 commit removing third_party/googletest_restricted_for_disagg_only

Make new branch spm-2926-gtest-staging, which is:
spm-2926-gtest-staging-base as base
1 or more commits of squashed work and/or rebased work.

Make a PR from spm-2926-gtest-staging.
Get approvals in the conventional way.

We can keep the PR updated with subsequent merges for as long as necessary.