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.