rpc: make generatetoaddress locals const

Summary:
and named the same as in generatetodescriptor just above.

This is a backport of core#19133 [2/10]
https://github.com/bitcoin/bitcoin/pull/19133/commits/9be7fd35c5d631c2cc34d3b4fa63ae0a9d5a68ef

Backport note: we use get_int64 instead of get_int for max_tries because of D7138

Depends on D9941

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9942