[7.1.0] Ensure that the mtime of an AC entry is smaller, not larger, than the...

[7.1.0] Ensure that the mtime of an AC entry is smaller, not larger, than the CAS blobs it references. (#21416)

https://github.com/bazelbuild/bazel/commit/7a774ff7c5850e7bd57af840a0e097ad4ec25417
got the condition backwards. Note that referential integrity isn't
required for correct operation, but lack of it can slow things down
unnecessarily.

PiperOrigin-RevId: 608354674
Change-Id: If56bb9211a5167d5db9e8eceb6224269758acdb1