rpc: Make verifychain default values static, not depend on global args

Summary:

This fixes several issues:

• The documentation is not compile-time static and depends on run-time arguments, making it impossible to host it on a static resource like a website or pdf. See also a similar change in the wallet rpc code: #18499
• The same call (relying on default values) will run different code on different machines, depending on the command line args that were used to start the server. This might lead to hard-to-debug-remote issues.

This is a backport of Core PR18541

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8911