Use Roslyn components multi-targeting

[Youssef1313]

We're using an old version of Roslyn to allow consumption in old VS versions. But this comes with some issues where a given Roslyn API had a bug that was fixed in a later version, and updating to new version of Roslyn is blocked.

Another benefit is we'll have more up-to-date nullable annotations, which can help prevent bugs (e.g #5802).

As of dotnet/sdk#20793, there is now a support for targeting multiple versions of Roslyn, where the highest version that can work will be picked. I think it would be useful if we used that in roslyn-analyzers.

[Youssef1313]

YairHalberstadt/stronginject@40c2721 for an example where this was done.

[Youssef1313]

This may help reduce the amount of reflection we do, ie, in Lightup we'll need to only use reflection if the API we need isn't available. That seems like a good addition. @mavasani What do you think?

[Youssef1313]

Actually we might completely get rid of Lightup with this.

[sharwell]

There are two big problems with Roslyn component versioning today:

1. All projects prior to SDK 6.0.100 do not understand component versions, and will include all versions of the assembly in the build. Supporting multi-targeting while also supporting Roslyn 3.x or earlier requires custom build logic inside the analyzer package.
2. Only SDK-style projects define SupportsRoslynComponentVersioning, so build logic relying on this property to work around (1) will force new projects to use older analyzers if they aren't SDK-style projects

[Youssef1313]

@sharwell I thought the SDK version requirement would be for roslyn-analyzers repository, not consumers. But looks like my assumption was wrong?

[sharwell]

Component versioning is a consumer/client feature implemented by NuGet and/or the SDK.

[Youssef1313]

@sharwell Is it expected that it can (at some point in future) be a good fit to use? If it's not a good fit, we can probably close this issue?

[sharwell]

I guess it depends on what versions of the compiler need to be supported over time. There are some partial workarounds available to handle cases where the feature is/is not supported.