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 [[https://github.com/bitcoin/bitcoin/pull/13676 | core#13676]] and [[https://github.com/bitcoin/bitcoin/pull/28358 | core#28358]]