Deprecate old ExecutionCache file formats
Created by: mrnugget
This fixes sourcegraph/sourcegraph#17306 by deleting support for old cache files.
The ticket is slightly wrong, since the previous code did already delete these files, so we don't have to deprecate and then delete support. We already transitioned to the new cache file format in 3.24 and 3.25 now.
The next release, 3.26, will only read the new cache format.