Configure validity dates with shorthands instead of start/end

@AGWA suggests at https://groups.google.com/a/chromium.org/g/ct-policy/c/936lR3MEUDU/m/fD9nWJceAgAJ

> I can't help but wonder if the weird start/end dates contributed to this
> mistake. When I was configuring my monitor, I was so focused on the
> month and day that I didn't notice the incorrect year and ended up
> misconfiguring it as 2025. (I only became aware of the mistake this
> morning when I received alerts about the log containing certificates
> outside of the range.) If logs generally configured their shards using
> strings like "2025", "2025h2" or "2025q3" instead of a pair of
> date-times, it seems like mistakes would be less likely.

No files, tests, or entry points are identified in the issue. Start by locating the existing validity-date configuration and reviewing the linked CT Policy discussion, then determine the shorthand formats, compatibility requirements, validation rules, and tests needed before considering the feature complete.

Written by the indexing model from the issue text.