`missing_safety_doc` accept uppercase "SAFETY" (#13701)

changelog: [`missing_safety_doc`]: accept uppercase "SAFETY"

In [Oxc](https://github.com/oxc-project/oxc)'s codebase, we try to draw
attention as clearly as possible to the safety constraints of unsafe
code by including an uppercase `# SAFETY` doc comment.

Clippy's `missing_safety_doc` lint does not recognise "SAFETY" in upper
case, so we also need to include `#[expect(clippy::missing_safety_doc)]`
on every unsafe function, to avoid a false positive. Unfortunately this
defeats the purpose of the lint, as if someone later removes the safety
docs, the lint rule does not trigger.

I don't know how common this style of documenting unsafe functions is,
but I don't imagine also supporting `# SAFETY` would disturb other users
who prefer `# Safety`.

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