Add the mtime fallback back (#97268)
Fixes
https://vercel.slack.com/archives/C08V7PA05D4/p1786563625559519?thread_ts=1786557472.558379&cid=C08V7PA05D4
Basically when trying to figure out how old a version is we parse the
CURRENT file, for an old version this fails. This PR adds a fallback for
mtime and a test
This fixes a bug introduced in #96941 which added the new TTL policy
