[source-build-release] Automate verification of expected release tags

[NikolaMilosavljevic]

In checklist's step 3.e, as a trigger for source-build release process ,we need to verify that 6.0 and 7.0 tags exist in public repos.

There might be a way to automate this so it is integrated in the release pipeline, and aids in usability.

[oleksandr-didyk]

This is already being done by the Validate release tag step in the Release stage. Example of this in the February release run for 6.0: run

There is still a small edge-case where if the release type is manually set to draft the tag verification will be skipped for servicing pre-VMR runs. Will fix it

[oleksandr-didyk]

PR - https://dev.azure.com/dnceng/internal/_git/dotnet-release/pullrequest/37553