Lowercase TargetOS

[ViktorHofer]

Follow-up on #32451 (comment)
Follow-up on #80074

In #43651, the "Windows" OS name was lowercased to achieve target frameworks with RIDs in them. At that time, the original plan was to lowercase all TargetOS values but that had to be cut because of resources. This PR finishes that and updates build systems, YML, markdowns and managed components.

To not break existing developer workflows, the passed-in -os value is automatically lower-cased in the script entry points and validated in the msbuild entry point.

This change was performed via

1. Manual "Find & Replace"**: <OS> -> <os> for all the possible OS values that we support (i.e. Browser -> browser) in msbuild, cmake, docs, etc.
2. Trial & Error to fix build failures
3. Looking through all the commits that went in between the Find & Replace was performed and now (Jan 10th) and performing the same operation again

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[ViktorHofer]

/azp list

[azure-pipelines]

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• runtime-coreclr gcstress-extra
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• runtime-coreclr jitstress-isas-x86
• runtime-coreclr jitstress-isas-arm
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• runtime-coreclr libraries-jitstress2-jitstressregs
• runtime-coreclr r2r
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• runtime-libraries-coreclr outerloop
• runtime-libraries-coreclr outerloop-windows
• runtime-libraries-coreclr outerloop-linux
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• runtime
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• coreclr-release-outerloop-nightly
• runtime-coreclr crossgen2-composite
• runtime-jit-experimental
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• dotnet-linker-tests
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• runtime-staging
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• Fuzzlyn
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• runtime-extra-platforms
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• runtime-android-grpc-client-tests
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• runtime-coreclr pgostress
• runtime-coreclr jitstress-isas-avx512
• runtime-tools-tests
• runtime-libraries-mono outerloop

[ViktorHofer]

/azp run runtime-extra-platforms

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[EgorBo]

Just updated my local main:

I assume it needs git clean -ffddxx ?

[ViktorHofer]

Sorry for the inconvenience and thanks for reporting. Just submitted a fix: #80544