Minimize diffs between main and nightly branches

[lbussell]

There are several places where main and nightly branches diverge today. Many of them could be reduced. This would reduce the overhead in maintaining this repo by reducing merge conflicts when porting changes between branches. It would also make assesing main-nightly diffs easier (better signal-to-noise ratio).

Area of Divergence	Can be removed	Notes
Content generated from templates	❌	by design
Versions in manifest.versions.json	❌	by design
Latest tags and floating tags in manifest.json	❌	limitation of ImageBuilder
mcr-tags-metadata-templates	❌	limitation of ImageBuilder
eng/dockerfile-templates/yarp	✅	YARP templates won't be used in main, so they can be added without negative side-effects
tests/ - YARP	✅	YARP test changes can be added without negative side effects since they won't run
tests/.../TestData.cs	✅	Some TestData (Alpha images, YARP, etc.) should be conditional on the nightly branch
eng/readme-templates/	✅	Missing improvements that were made when adding YARP images
eng/readme-templates/ - YARP	✅	YARP readme templates won't be used in main branch
eng/pipelines/	✅	Some templates were removed in nightly and not in main