Disable the Platform Compat Analyzer until #41354 is fixed

[jeffhandley]

We intend to merge the Platform Compatibility Analyzer into the .NET 5.0 RC1 SDK. When the updated version of the NetAnalyzers package is taken into the dotnet/runtime repo that contains this analyzer, it will introduce build warnings. Those build warnings surface because:

• The Runtime repo applies [SupportedOSPlatform] and [UnsupportedOSPlatform] attributes to many APIs now
• We began applying those attributes before having the analyzer merged in because we wanted to have those attributes in place for the RC1 release
• There are cases, specifically around our multi-targeting, where we will need to either apply further annotations or utilize guard methods around calls to the platform-specific APIs

Because these warnings will occur, we are going to disable the analyzer in the dotnet/runtime repo before merging it into the SDK. Before we ship .NET 5.0 RC2 though, we want to re-enable the analyzer and address all of the warnings introduced. #41354 tracks that work.

[buyaa-n]

Thanks!

[ghost]

Hello @jeffhandley!

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[jeffhandley]

@safern Do you see any reason to port this into release/5.0? So long as we don't update the NetAnalyzers package version in that branch, the new analyzer won't be applied.

[safern]

@jeffhandley I would say let's not port it and if for some reason we need to update the analyzer's package into release/5.0 branch then we can disable this rule as part of that PR.