Update build instructions for windows

[MaStr11]

Since #22812 roslyn only builds with VS 15.5 Preview. The build instruction for windows should be updated:

• Minimum Version is 15.5 Preview
• Required SDK is 2.1.1 (couldn't find any download link for this)
• SDK 2.1.1 installs with VS 15.5 Preview

[sharwell]

@dotnet/roslyn-compiler We should make sure to track this as a key part of updating the compiler version since it's likely to happen again in the future.

[jasonmalinowski]

Oops, that was actually an oversight: we intentionally didn't want master to have that dependency until 15.5 shipped in a non-Preview form.

[jaredpar]

The changes for the SDK links and versions is already in PR.

#23494

Oops, that was actually an oversight: we intentionally didn't want master to have that dependency until 15.5 shipped in a non-Preview form.

Agree. We did this work initially in the post-dev15.5-contrib branch. The assumption was that wouldn't be merged until 15.5 was public. That turned out to not be true though.