[msbuild] Make the level of parallelism configurable in all tasks that call Parallel.ForEach. Fixes #20210.

[rolfbjarne]

This deduplicates some code, and also ensures we specifiy a max limit to the
level of parallelism. This fixes an issue in the AOTCompile and ScnTool tasks,
where we'd have no limit, launching as many subprocesses as tasks there are to
execute. In particular for the AOTCompile task, this can put a rather heavy
burden on build machines, slowing everything down to a crawl.

Fixes #20210.

[vs-mobiletools-engineering-service2]

💻 [PR Build] Tests on macOS X64 - Mac Sonoma (14) passed 💻

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💻 [PR Build] Tests on macOS M1 - Mac Big Sur (11) passed 💻

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• xamarin.ios-17.3.0.242.pkg
• xamarin.mac-9.3.0.242.pkg

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💻 [PR Build] Tests on macOS M1 - Mac Ventura (13) passed 💻

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💻 [CI Build] Windows Integration Tests passed 💻

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Hash: 318b28509f4b48d29fb5dfd412ab95790a11d94a [PR build]

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✅ API diff for current PR / commit

Legacy Xamarin (No breaking changes)

• iOS (no change detected)
• tvOS (no change detected)
• watchOS (no change detected)
• macOS (no change detected)

NET (empty diffs)

• iOS: (empty diff detected)
• tvOS: (empty diff detected)
• MacCatalyst: (empty diff detected)
• macOS: (empty diff detected)

✅ API diff vs stable

Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)
• Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

Legacy Xamarin (stable) vs .NET

• iOS: vsdrops gist
• tvOS: vsdrops gist
• macOS: vsdrops gist

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

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[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

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✅ All tests passed on VSTS: test results.

🎉 All 170 tests passed 🎉

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✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 8 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 7 tests passed. Html Report (VSDrops) Download
✅ generator: All 2 tests passed. Html Report (VSDrops) Download
✅ install-source: All 1 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 7 tests passed. Html Report (VSDrops) Download
✅ introspection: All 8 tests passed. Html Report (VSDrops) Download
✅ linker: All 65 tests passed. Html Report (VSDrops) Download
✅ mac-binding-project: All 1 tests passed. Html Report (VSDrops) Download
✅ mmp: All 2 tests passed. Html Report (VSDrops) Download
✅ mononative: All 6 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 7 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 8 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (watchOS): All 4 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ mtouch: All 1 tests passed. Html Report (VSDrops) Download
✅ xammac: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 8 tests passed. Html Report (VSDrops) Download
✅ xtro: All 2 tests passed. Html Report (VSDrops) Download

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Hash: 318b28509f4b48d29fb5dfd412ab95790a11d94a [PR build]