Implement BIP 8 based Speedy Trial activation

Implements the block height (BIP 8) based Speedy Trial activation proposal that was discussed on the bitcoin-dev mailing list.

In order to do so, this PR first changes the versionbits (BIP 9) implementation to use heights rather than MTP. Then the minimum activation height parameter is introduced. This also adds the minimum activation height as a third parameter of -vbparams. Additionally, the threshold is made a parameter of each deployment so that it can be customized for each deployment. Lastly several unit and functional tests are added.

In order to make this easier to backport, the names of functions and classes are largely unchanged (except where necessary for the height based things). This means that some things are misnamed as they refer to BIP 9 even though this is really BIP 8.

This PR borrows 2 commits from #19573 to implement height based versionbits, one commit from #21377 for tests, and 1 commit formerly part of #21380 for the threshold parameterization.