[release/8.0-preview2] Versions.props: Increment package versions for…

… 6.0, and 7.0 (dotnet#82735)

* 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

Force building of Mono manifest packages when building via source-build

Backport of dotnet#81790

Merge in 'release/6.0' changes

Merged PR 29014: Disable source build for 7.0 in official and revert …

…PR disabling