Only support 32-byte keys in ChaCha20{,Aligned}
Summary:
This is a partial backport of core#26153
https://github.com/bitcoin/bitcoin/pull/26153/commits/62ec713961ade7b58e90c905395558a41e8a59f0
Depends on D18823
Test Plan: ninja all check-all bench-bitcoin bitcoin-fuzzers
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D18824