[rpc] changed MineBlocksOnDemand to IsMockableChain

Summary:

Change: Update the if statement in setmocktime to use IsMockableChain chainparams function (aka m_is_mockable_chain) instead of MineBlocksOnDemand

Rationale: It's a more appropriate check for whether or not chain is in RegTest, as discussed in PR18037

This is a backport of Core PR18263

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8783