rpc: Uncouple non-wallet rpcs from maxTxFee global

Summary:

• rpc: Use IsValidNumArgs over hardcoded size checks
• rpc: Uncouple rpcs from maxTxFee global
• doc: Add release notes for 15620

This is a backport of Core PR15620

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6266