Merge bitcoin/bitcoin#28358: Drop -dbcache limit

bb3b980dfd validation: drop maximum -dbcache (Sjors Provoost)

Pull request description:

  Due to recent UTXO set growth, the current maximum value for `-dbcache` of 16GB is ~just months away from being~ 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.

  Fixes #28249.

  ---

  A previous version of this PR increased the maximum to 64GB. It also made startup abort if the value provided is too high, rather than quietly round it down. But this didn't get much support.

ACKs for top commit:
  achow101:
    ACK bb3b980dfd
  tdb3:
    ACK bb3b980dfd
  BenWestgate:
    crACK bb3b980dfd.

Tree-SHA512: 8515fff468c2387a0b04bd9523ab1df46d6325738588b7550fabddbb8624817a583d95b95ea246407f9f0ff3e43e760cf7334621bec6af79710176328528a3ef

doc/release-notes-28358.md

@@ -0,0 +1,6 @@
+Updated settings
+------
+
+- The maximum allowed value for the `-dbcache` configuration option has been
+  dropped due to recent UTXO set growth. Note that before this change, large `-dbcache`
+  values were automatically reduced to 16 GiB (1 GiB on 32 bit systems). (#28358)