Deduplicate last-continuous and last-lts multiversion tasks if the versions are identical

Since 5.0 is both last-its and last-continuous, we should not need to run both by proactively checking that the versions are identical in multiversionconstants.py

Exclusions in backports_required_multiversion.yml only needed to be added to the last-lts section when it's equal to last-continuous. Since the next time last-continuous is updated, the last-continuous section should be cleared out.

For future lts releases, last-lts and last-continuous would be identical. We therefore only run multiversion tests once.

The last-lts and last-continuous check can be done by checking multiversionconstants.py