Rename applicability levels to always, sometimes, and never (#7821)

Following much discussion for #4181 at
https://github.com/astral-sh/ruff/pull/5119,
https://github.com/astral-sh/ruff/discussions/5476, #7769,
https://github.com/astral-sh/ruff/pull/7819, and in
[Discord](1159144114),
this pull request changes `Applicability` from using `Automatic`,
`Suggested`, and `Manual` to `Always`, `Sometimes`, and `Never`.

Also removes `Applicability::Unspecified` (replacing #7792).

crates/ruff_linter/src/fix/mod.rs

@@ -151,7 +151,7 @@ mod tests {
                 // The choice of rule here is arbitrary.
                 kind: MissingNewlineAtEndOfFile.into(),
                 range: edit.range(),
-                fix: Some(Fix::unspecified(edit)),
+                fix: Some(Fix::always_applies(edit)),
                 parent: None,
             })
             .collect()