Convert OldDiagnostic::noqa_code to an Option<String> (#18946)

## Summary

I think this should be the last step before combining `OldDiagnostic`
and `ruff_db::Diagnostic`. We can't store a `NoqaCode` on
`ruff_db::Diagnostic`, so I converted the `noqa_code` field to an
`Option<String>` and then propagated this change to all of the callers.

I tried to use `&str` everywhere it was possible, so I think the
remaining `to_string` calls are necessary. I spent some time trying to
convert _everything_ to `&str` but ran into lifetime issues, especially
in the `FixTable`. Maybe we can take another look at that if it causes a
performance regression, but hopefully these paths aren't too hot. We
also avoid some `to_string` calls, so it might even out a bit too.

## Test Plan

Existing tests

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_linter/src/codes.rs

@@ -46,6 +46,12 @@ impl PartialEq<&str> for NoqaCode {
     }
 }
 
+impl PartialEq<NoqaCode> for &str {
+    fn eq(&self, other: &NoqaCode) -> bool {
+        other.eq(self)
+    }
+}
+
 impl serde::Serialize for NoqaCode {
     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
     where