(#22608) Restore the ability for master to accept pson file backups

Commit 769dcdf for #22375 removed the ability for newer masters to
accept pson encoded file backups from older agents.

This commit ensures that masters can deserialize `s` or `pson` encoded file
backups, while agents will serialize using `s` by default.

It also restores the ability to serialize to pson, but adds a
deprecation warning if that happens. Since pson is not the
`default_format` the agent will never serialize file bucket files to
pson.