bump WarningLevel to 7 for .NET 7

[baronfel]

The C# compiler supports a warning wave of 7 now, and we should float that on like was done for .NET 6. This PR does that, in a slightly more maintainable way IMO.

The rules are:

• AnalysisLevel should track the current TFM if not specifed
• AnalysisLevel should be set to latest if the current TFM is the 'latest' TFM (as defined by a property that we need to bump)
• WarningLevel should track the current TFM if not specified
• WarningLevel shouldn't override the user-provided value
• WarningLevel should be set to 4 if the project is a .NET Framework project

Fixes #21599 as well

cc @Youssef1313 via dotnet/templating#4580

[Youssef1313]

While the change on SDK side is needed, it doesn't fully resolve the suggestion for templates.

A user might not specify AnalysisLevel or might specify low value for AnalysisLevel to get a smaller set of CAxxxx analyzers to run (due to FPs, rules that user think are noisy, too much new warnings after update, etc).

For compiler warnings, they're almost always correct warnings that users are encouraged to fix.

[baronfel]

I would love to add a test that computes the uses TFM of the test project to assert that the latest and preview AnalysisLevels (and the warning level) float to the TFMs as we update them. Ideally the test would fail as soon as the TFM is changed from net7.0 to net8.0, so that we know that some update here has to be done.

[cremor]

@baronfel

WarningLevel should be set to 4 if the project is a .NET Framework project

Why is this rule/condition on the framework type and not on the C# language version?

If I remember correctly, my .NET Framework project which uses an SDK-style csproj file with <LangVersion>10.0</LangVersion> and <AnalysisLevel>latest</AnalysisLevel> used warning level 6 with the .NET 6 SDK. Now with the .NET 7 SDK it only uses warning level 4 again.

Yes, I can explicitly specify the <WarningLevel>, but shouldn't setting <LangVersion> and <AnalysisLevel> be enough to say "I want the latest compiler features and warnings"?

Btw, this also affects documentation, see dotnet/docs#33559

[sliekens]

@baronfel I think this PR introduced a regression.

Before this PR, the WarningLevel could be set to match the version number of the EffectiveAnalysisLevel, but this code was removed. Now, setting AnalysisLevel=preview still results in WarnLevel=9999 but any other value for AnalysisLevel will be ignored and the effective WarnLevel is derived from the TFM.

First I thought this was a documentation issue.
dotnet/docs#41295

Now I believe it is a regression.