feat: Add .NET 9 support (and drop .NET 6 support)

[filzrev]

This PR including following changes.

• Add .NET 9 support (.NET 9,0 RC is already released as go-live)
• Drop .NET 6 support (Support ends at November 12, 2024)

Note:
The SDK for Self-Contained exe zips will remain .NET 8.
Because it's .NET 8 is LTS release and longer support lifetime.

[paulushub]

For a tool, is there any real need to even drop support for the framework before the support ends?
Many applications and libraries will still support the framework after the official end of support.

[filzrev]

I'm expecting this PR is merged and released newer docfx version after .NET 9 RTM release.
(or .NET 9 version of Roslyn packages are released).

The reason I've dropped .NET 6 supports is following.

1. Microsoft.CodeAnalysis.Workspaces.MSBuild supports only latest 2 versions (.NET 6 support is already dropped)
   So when continuing .NET 6 supports. It need to reference older versions and it introduce additional complexity.
2. Published NuGet tool package size issue (Exploded package size and quality concerns #9396).
   .NET tools package contains files per TargetFramework.
   So it increase package size when continuing support .NET 6.
3. Local disk usage issue
   Currently when building/publishing on local dev environment. It consume a lot of disks spaces.
   (On my environment it takes over 10GB disk spaces).

[paulushub]

1. Microsoft.CodeAnalysis.Workspaces.MSBuild supports only latest 2 versions (.NET 6 support is already dropped)

If I understand it well, this normally should not be an issue. Reference the version that supports the .NET 6 for the .NET 6 build.

2. Published NuGet tool package size issue ([Exploded package size and quality concerns Exploded package size and quality concerns #9396]
3. Local disk usage issue
   Currently when building/publishing on local dev environment. It consume a lot of disks spaces.
   (On my environment it takes over 10GB disk spaces).

Just updated my version to 2.77.0 and here are the stats of the installation:
Size: 0.99 GB (1,063,444,125 bytes)
Size on disk: 0.99 GB (1,065,295,872 bytes)
Contains: 969 Files, 113 Folders

If data are later downloaded, I will assume these are not Framework dependent. Again, I might be missing something.

[yufeih]

I agree with dropping unsupported .NET versions to reduce unneeded maintainance effort. This will to be merged and released after November 12, 2024.

[filzrev]

3. Local disk usage issue

Please ignore the above comments.
It's build environment issues. and this problem is now mitigated by #10066.

For docfx users. keeping .NET 6 support requires about 37MB extra local disk spaces. and increase about 16MB on nupkg file size.

In any case, I would like to drop .NET 6 support to lower maintenance costs with following reasons

• It takes extra LocalBuild/CI test time.
• Remove Conditional NuGet package references (for Roslyn APIs)
• Reduce #ifdef directive usage
• Using newer .NET APIs (e.g. span-based overload, Frozen Collection)

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For users who continue to using .NET 6. It need to use version that supports .NET 6(2.77.0)