validation: drop maximum -dbcache

Summary:
If the UTXO set grows, the maximum value for -dbcache may become insufficient for those who wish to complete IBD with the UTXO set held in RAM.

This drops the limit. It also adds a warning that it's up to users to check that they have enough RAM.

This is a backport of core#13676 and core#28358

Depends on D18575

Test Plan:
ninja all check-all

$ src/bitcoind -dbcache=32000
...
2025-09-09T13:43:09Z * Using 31990.0 MiB for in-memory UTXO set (plus up to 286.1 MiB of unused mempool space)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18577