[QUIC] Add platform guard to IsSupported

[ManickaP]

This should make the IsSupported properties not trigger the compiler warning about using unsupported API, in other words avoiding this:

warning CA1416: This call site is reachable on all platforms. 'QuicListener.IsSupported' is only supported on: 'linux', 'macOS/OSX', 'windows'. (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1416)

for a simple:

if (QuicListener.IsSupported)
{
    ...
}

We might even try if we could get away with using the QuicConnection.IsSupported as a trimming guard (instead of IsHttp3Supported()) in HTTP code.

EDIT: Analyzer is in main. PR as a draft for now since it depends on dotnet/roslyn-analyzers#7326

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[rzikm]

LGTM

[buyaa-n]

Probably no need to add the attributes into generated files (QuicConnection.Unsupported.cs, QuicListener.Unsupported.cs), anyway if/when they regenerated the attributes will not be there.

[ManickaP]

generated files (QuicConnection.Unsupported.cs, QuicListener.Unsupported.cs)

They're not generated though, their content is on PNSE exclusion list and these sources are used instead.

[ManickaP]

/ba-g wasm failure reported here: #103623