GH-70647: Deprecate strptime day of month parsing without a year present to avoid leap-year bugs

[gpshead]

If we don't do this now, people will still be using Python releases that don't guide code away from the bug by the time the next leap year rolls around. :)

Decision on what our actual breaking change will be has been left for later, this just starts the process to allow us to do it with a suggestion of how to always avoid such problems in code.

TODO list:

• [ ] add documentation changes to the PR. (beyond what #116179 does as that'll be backported, document the specific deprecation in this PR)

• Issue: gh-70647