Yml: Use rids instead of "Platform"

[ViktorHofer]

Currently we use a "Platform" layout instead of RID. Except for the delimiter and the casing, both formats are identical:

      targetRid: ios-arm64
      platform: iOS_arm64

Unfortunately for Windows this isn't true:

      targetRid: win-arm
      platform: Windows_NT_arm

Mono will publish the runtime targeting packs which contain the rid in the directory name as part of libraries's build-job.yml, hence the RID needs to be available. I suggest that we replace the made-up "Platform" format with the RID layout.

cc @safern @premun

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[safern]

I think this makes sense, but maybe should be part of: #32451 ?