refactor: Add and use PRUNE_TARGET_MANUAL constexpr

Summary:
This concludes backport of core#25781
https://github.com/bitcoin/bitcoin/pull/25781/commits/fadf8b818226dc60adf88e927160f9c9680473a4

Depends on D16009

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16010