Rename ChaCha20::Seek -> Seek64 to clarify multiple of 64
Summary:
This is a partial backport of core#26153
https://github.com/bitcoin/bitcoin/pull/26153/commits/6babf402130a8f3ef3058594750aeaa50b8f5044
Depends on D18817
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/D18818