Automate source-build release process for 6.0 and 7.0

[lbussell]

Describe the Problem

The source-build release process takes up a lot of time. Also, many of the release tasks are completed manually which leaves lots of room for individual error.

Describe the Solution

The current release process is documented here: https://github.com/dotnet/source-build/blob/3c8f23d08475e6f59a5ca889e0719b61e18db476/.github/ISSUE_TEMPLATE/releases/release_checklist.md

The large majority of those release tasks should be automated. We can break this up into pipelines that can be run during different parts of the release cycle. I am proposing that we expand the current source-build-release pipeline and create a new pipeline, source-build-pre-release.

source-build-pre-release (in order of execution, not priority):

• Automate the publishing process of source tarballs for servicing releases #2710
• Automate the package collection process to satisfy pre-release smoke testing requirements #3068
• Automatically create new VMR-lite branches in source-build-release pipeline #3064
• Automatically submit update PRs to VMR-lite #3069
• Implement CI for servicing VMR #2904

In between pre-release and release, after security-partners-dotnet PRs are merged:

• Mirror dnceng/security-partners-dotnet release/* branches to dotnet-security-partners/dotnet #3147
• Automatically tag releases in dotnet-security-partners/dotnet repo #3179
• Automate source-build partners content #3196

source-build-release:

• Automatically upload Private.SourceBuilt.Artifacts to blob storage on release day #3080
• Automate SDK comparison baseline updates #3062
• Include machine-readable info in markdown comment in release announcement post #2888
• Source build release pipeline should validate the availability of the commit shas #3058

[lbussell]

In terms of priority, I see the following as most important:

• Automatically create new VMR-lite branches in source-build-release pipeline #3064: Updating the VMR-lite is done manually and prone to individual error
• Implement CI for servicing VMR #2904: We currently only manually test the VMR-lite

Everything else is fairly well defined, just manual work.

[MichaelSimons]

@lbussell - where does #2921 fit in here? Is it a duplicate of #3096?