Update `non_exhaustive` to use the attribute template

[ehuss]

New rules:

• ❗ attributes.type-system.non_exhaustive.duplicates
• ❗ attributes.type-system.non_exhaustive.enum-exhaustiveness
• ❗ attributes.type-system.non_exhaustive.enum-cast
• ❗ attributes.type-system.non_exhaustive.inhabited

Renamed rules:

• attributes.type-system.non_exhaustive.external-crate moved to .intro because it is generally explaining the purpose.

[rustbot]

:umbrella: The latest upstream changes (possibly #2015) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.