Remove references to "new" features introduced before C# 9

[BillWagner]

Fixes #30422

Several locations in our docs refer to features as "new" and reference the version when introduced. This is great for the newest features, but doesn't enhance the experience for features introduced in versions as old as C# 2.

Each commit in this PR goes through C# versions starting with C# 2.0, removing language that references when a feature was introduced. The latest version where this language was removed is C# 8.0.

The exceptions are articles on compiler errors and warnings, where version differences are important, and articles that are specifically about C# version history.