miscellaneous pg_upgrade cleanup

Here are a few miscellaneous cleanup patches for pg_upgrade. I don't think
there's anything controversial here.

0001 removes some extra whitespace in the status message for failed data
type checks. I noticed that when the check fails, this status message is
indented beyond all the other output. This appears to have been introduced
in commit 347758b, so I'd back-patch this one to v17.

0002 improves the coding style in many of the new upgrade task callback
functions. I refrained from adjusting this code too much when converting
these tasks to use the new pg_upgrade task framework (see commit 40e2e5e),
but now I think we should. This decreases the amount of indentation in
some places and removes a few dozen lines of code.

0003 adds names to the UpgradeTaskSlotState enum and the UpgradeTaskSlot
struct. I'm not aware of any established project policy in this area, but
I figured it'd be good to at least be consistent within the same file.

Thoughts?

--
nathan