Revert "Bump minimum toolchain version"

A handful of bots are still breaking, either because I missed them in my audit,
they were offline, or something else. I'm contacting their authors, but I'll
revert for now and re-commit later.

llvm-svn: 352814
GitOrigin-RevId: e2dedd55645cf035ee43cf54bea176f61d587252
4 files changed