Analyzer proposal: Catch uses of older VisualStudioServices version

[AArnott]

Problem statement

Folks who target 16.9 but use VisualStudioServices.VS2019_7 are missing out on functionality or perf improvements that may be reserved to users of VisualStudioServices.VS2019_9.

Proposed solution

As existing code from when code targeted an older version is inevitable, we should add an analyzer that notices when any version older than the most recent available one is used given the reference assemblies.

I suggest making the diagnostic severity default to Info, since light-up scenarios or behavioral difference between service moniker versions can make upgrading significant.

A code fix could offer to bulk update all uses too.