[rpc] changed MineBlocksOnDemand to IsMockableChain
[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 [[https://github.com/bitcoin/bitcoin/pull/18037 | PR18037]]

This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/18263 | PR18263]]

Test Plan: `ninja all check-all`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

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

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