Add an MSRV note for retain_mut

(cherry picked from commit 6b425e4)

src/map/core.rs

@@ -399,6 +399,7 @@ impl<K, V> IndexMapCore<K, V> {
  where
  F: FnMut(&mut K, &mut V) -> bool,
  {
+ // FIXME: This could use Vec::retain_mut with MSRV 1.61.
  // Like Vec::retain in self.entries, but with mutable K and V.
  // We swap-shift all the items we want to keep, truncate the rest,
  // then rebuild the raw hash table with the new indexes.