Automatically submit update PRs to VMR-lite

[lbussell]

Requires #3064 as a prerequisite.

After we have an updated branch, we should create a PR to the correct internal branch using the Azure DevOps API.

This is relatively low priority because the return on investment is low, but it's necessary for complete release automation. It's very easy to submit PRs manually via the web UI.

[crummel]

I believe this will be covered by @premun's unified release pipeline work, nominating for closure.

[premun]

Just for clarity - I think this is worth doing but my changes to the pipelines are orthogonal to this and I wasn't planning to prioritize this. Not sure I'll have the cycles.