* Add support for skipping workloads testing for previous versions

This will skip installing manifests for 6, and 7, for example, and
install only install the latest ones. This is because on release
branches the newest versions might not have public packages available
yet.

* Set $(WorkloadsTestPreviousVersions) in the yml

.. based on the branch. Set it to `true` only for `main`.

* Versions.props: Increment package versions for 6.0, and 7.0

* Set WorkloadsTestPreviousVersions=true by default

* Set default for WorkloadsTestPreviousVersions