Merge bitcoin/bitcoin#20867: Support up to 20 keys for multisig under Segwit context

ebd4be43cc doc: add release notes for 20867 (Antoine Poinsot)
5aa50ab9cc rpc/util: multisig: only check redeemScript size is <= 520 for P2SH (Antoine Poinsot)
063df9e897 test/functional: standardness sanity checks for P2(W)SH multisig (Antoine Poinsot)
ae0429d3af script: allow up to 20 keys in wsh() descriptors (Antoine Poinsot)
9fc68faf35 script: match multisigs with up to MAX_PUBKEYS_PER_MULTISIG keys (Antoine Poinsot)

Pull request description:

  As described in https://github.com/bitcoin/bitcoin/issues/20620 multisigs are currently limited to 16 keys in descriptors and RPC helpers, even for P2WSH and P2SH-P2WSH.

  This adds support for multisig with up to 20 keys (which are already standard) for Segwit v0 context for descriptors (`wsh()`, `sh(wsh())`) and RPC helpers.

  Fixes https://github.com/bitcoin/bitcoin/issues/20620

ACKs for top commit:
  meshcollider:
    re-utACK ebd4be43cc
  instagibbs:
    re-ACK https://github.com/bitcoin/bitcoin/pull/20867/commits/ebd4be43cc945e643f91d3a91007b5a35bbbd5a1

Tree-SHA512: 36141f10a8288010d17d5c4fe8d24878bcd4533b88a8aba3a44fa8f74ceb3182d70fee01427e0ab7f53ce7fab46c88c1cd3ac3b18ab8a10bd4a6b8b74ed79e46

doc/release-notes-20867.md

@@ -0,0 +1,11 @@
+Wallet
+------
+
+- We now support up to 20 keys in `multi()` and `sortedmulti()` descriptors
+  under `wsh()`. (#20867)
+
+Updated RPCs
+------------
+
+- `addmultisigaddress` and `createmultisig` now support up to 20 keys for
+  Segwit addresses.