Rename SdkPackageUpdater => PackageReferenceUpdater, for clarity

[rhyskoedijk]

What are you trying to accomplish?

I'm looking at how #10260 can be fixed, but the current terminology is a little confusing; It would be good to clear this up.

The change adds documentation to clarify the scenarios where a "packages.config" update and "PackageReference" update could happen. The current terminology, "SdkPackageUpdater", gives the impression that "PackageReference" will only appear in SDK-style project files, which is not correct (see code comments).

Anything you want to highlight for special attention from reviewers?

No changes to behaviour, purely documentation.

How will you know you've accomplished your goal?

I can easily identify which updater class is applicable to each project file format.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[brettfo]

@rhyskoedijk A change was just merged that edited two of the files that were renamed, causing a conflict. Could you resolve the conflicts and update this PR? I'll hold off other C# changes until this is in so we don't keep stepping on your toes.

[rhyskoedijk]

@brettfo I've updated the PR; one of the smoke tests failed due to a network error, is it possible to re-queue it?