Rollup merge of #141162 - mejrs:gated, r=fee1-dead

refactor  `AttributeGate` and `rustc_attr!` to emit notes during feature checking

First commit changes the following:

- `AttributeGate ` from an enum with (four) tuple fields to (five) named fields
- adds a `notes` fields that is emitted as notes in the `PostExpansionVisitor` pass
- removes the `this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date` note if the feature gate is `rustc_attrs`.
- various phrasing changes and touchups
- and finally, the reason why I went down this path to begin with: tell people they can use the diagnostic namespace when they hit the rustc_on_unimplemented feature gate 🙈

Second commit removes unused machinery for deprecated attributes

Author: matthiaskrgr