[Version-unspecific] Only halt after THOR height specified by HaltSigning and HaltSigning%s Mimir keys, like all other Halt keys

[Version unspecific]

Intended to close #2047 (closed)
'Bifrost should respect future height in HaltSigning'.

My current impression is that no versioning is needed for this.
(No observation consensus issues, nor predicted switchover-timing usage.)

My thought process code references for referring to the THOR height,
in that every other Halt key does so and HaltSigning has to (since it can't be set chain-specifically),
are here:
#2047 (comment 2083042237)

As always, feedback is welcome!