Remove unnecessary lifetimes on Buffer::diagnostic_group (#22698)

Release Notes:

- N/A

crates/language/src/buffer.rs

@@ -4019,12 +4019,12 @@ impl BufferSnapshot {
     }
 
     /// Returns an iterator over the diagnostics for the given group.
-    pub fn diagnostic_group<'a, O>(
+    pub fn diagnostic_group<O>(
-        &'a self,
+        &self,
         group_id: usize,
-    ) -> impl 'a + Iterator<Item = DiagnosticEntry<O>>
+    ) -> impl Iterator<Item = DiagnosticEntry<O>> + '_
     where
-        O: 'a + FromAnchor,
+        O: FromAnchor + 'static,
     {
         self.diagnostics
             .iter()