Remove support for deprecated leading new in comment references

[srawlins]

The syntax, [new Foo] and [Foo.Foo] have long been deprecated. The analyzer reports [new Foo] as well, suggesting to replace with [Foo.new]. This Change removes the support for the deprecated syntaxes.

All examples in tests have been changed to [Foo.new]. There are also tests for the disambiguation of a constructor and an instance field with the same name which is allowed), e.g. in [Foo.foo]. The supported disambiguation hint is to add trailing parentheses: [Foo.foo()] to indicate the constructor.

Fixes #3531

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[srawlins]

Blocked by https://dart-review.googlesource.com/c/sdk/+/330683.