[api] Update synth_mint_paused to Respect MintSynths Mimir

I sent an interface announcement reminder on this today - let's hold this for the next release and target 1.104.

See context in thread below - I had missed that there was a MintSynths mimir since isSynthMintPaused does not check it. Instead of removing this field from the response, this PR is updated to add the check on that mimir, which may be flagged off to prevent synth minting even when there is available capacity.