Use `Global_slot_since_genesis` for stop slots

[mrmr1993]

This PR is the correct fix for the consistency issue 'fixed' in https://github.com/minaprotocol/mina/pull/15503. The global slot since hard fork was incorrectly used in the original PR https://github.com/MinaProtocol/mina/pull/14516; we fix the issue by using the global slot since genesis instead, so that an emergency hard-fork post-berkeley will not force us to use an unrepresentative stop slot.

[mrmr1993]

!ci-build-me

[mrmr1993]

!ci-nightly-me