Update Shared State mirror to kirkstone and add Hash Equivalence

[t-mon]

This might fix the upstream sstate cache and also make use of upstream hash serve:
https://docs.yoctoproject.org/dev/migration-guides/release-notes-4.0.html

[mhei]

After re-checking, I wonder whether we should drop BB_SIGNATURE_HANDLER = "OEEquivHash" - it seems to be the default anyway?
And I'd assign BB_HASHSERVE and BB_HASHSERVE_UPSTREAM also with ?= instead of forcing the assignment.