This adds the `unloadwallet` RPC, release notes, and tests for it.
Partial backport of Core PR13111
https://github.com/bitcoin/bitcoin/pull/13111/commits/6608c369b1a6cfc1d5b4a7905c193baa999ba84c
https://github.com/bitcoin/bitcoin/pull/13111/commits/9f9b50d5feb1e604283c463e289e83b63a849a8c
https://github.com/bitcoin/bitcoin/pull/13111/commits/4940a20a46685cd56ea045d8cc7fe058c6222431
https://github.com/bitcoin/bitcoin/pull/13111/commits/ccbf7ae7496fd13b4147aa13d7408712bd90c614
https://github.com/bitcoin/bitcoin/pull/13111/commits/0b82bac76d0f842bd2294a290388536951fbc576 (only the additional test added to `wallet_multiwallet.py` is taken from here).
Depends on D4252