Configure validity dates with shorthands instead of start/end

[FiloSottile]

@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.