refactor: simplify pruning violation check

Summary:
By generalizing 'GetFirstStoredBlock' and implementing
'CheckBlockDataAvailability' we can dedup code and
avoid repeating work when multiple indexes are enabled.
E.g. get the oldest block across all indexes and
perform the pruning violation check from that point
up to the tip only once (this feature is being introduced
in a follow-up commit).

This commit shouldn't change behavior in any way.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

This is a partial backport of core#27607 and core#25060 (for LIFETIMEBOUND)
https://github.com/bitcoin/bitcoin/pull/27607/commits/2ec89f1970935d27631bcd17b7923a79cdb1edbb

Depends on D17747

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17748