[DO NOT MERGE] Switch to using darc vmr forwardflow

[premun]

dotnet/arcade-services#4539

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[premun]

@NikolaMilosavljevic I made a small fix in darc, bumped it here and now it's synchronizing using the darc vmr forwardflow command. But for your experiments with moving the templates, I'd suggest:

• Leaving the files changed in this PR more or less intact (keep using darc vmr update basically)
• Pick a different repo than dotnet/sdk to test the templates as the other repos don't have SB patches and additional mappings
• Focus on figuring out how to pull in the vmr-* YAML correctly and run the build

After you get this part, I'd do the switch of the command and we will also have to wait for the flat flow switch probably (and for first backflows into every repo).

[premun]

This is no longer needed because we removed the UB and SB legs