Update NuGet packages to 6.5.0

[dfederm]

This is a fairly low-pri but low-risk change. It updates from the RC packages to the GA ones.

The packages this applies to and their usages are:

• Microsoft.Build.NuGetSdkResolver is used by MSBuild.csproj and the bootstrap. For MSBuild.csproj only grabs an XML file from it (which points to the Microsoft.Build.NuGetSdkResolver.dll binary "2 directories up"; the actual binary ships separately), and is identical to the previous version.
• NuGet.Build.Tasks (and NuGet.Build.Tasks.Console once Fix static graph restore functionality for the bootstrap #8488 is merged) are only used in bootstrapping.
• NuGet.Frameworks is only used in UTs (Microsoft.Build.Engine.UnitTests.csproj)

[rainersigwald]

We do have automation for this but we've already moved the subscription to the 17.7 packages (and evidently forgot to move to 17.6 during that whole cycle, whoops). No objection to taking this but we should move forward as soon as NuGet publishes a 6.7 prerel package.

[rainersigwald]

Ugh, it's failing because the package isn't available in feeds. Do you actually need this for anything or just cleaning up? If the latter I'm inclined to just wait.

[dfederm]

Ugh, it's failing because the package isn't available in feeds. Do you actually need this for anything or just cleaning up? If the latter I'm inclined to just wait.

Nope, no specific need. I actually want some fixes in 6.6.x, but those aren't publicly available, and I figured I'd do this minor cleanup while I was here.

Didn't notice that the version for Microsoft.Build.NuGetSdkResolver is woefully behind on NuGet.org :(

[dfederm]

Closing in favor of waiting for 6.7 preview packages