rustdoc: Highlight edition-specific keywords correctly in code blocks, accounting for code block edition modifiers

This is a squash of these commits:
- Highlight edition-specific keywords correctly in code blocks,
accounting for code block edition modifiers
- Fix unit tests
- Revert changes to rustc_span::symbol to prepare for merge of #80272
- Use new Symbol::is_reserved API from #80272
- Remove unused import added by accident when merging

src/librustdoc/html/render/mod.rs

@@ -4747,6 +4747,7 @@ fn item_macro(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::Mac
             Some("macro"),
             None,
             None,
+            it.source.span().edition(),
         ))
     });
     document(w, cx, it, None)