release notes and gcc 15 filter out

Author: huixie90

libcxx/docs/ReleaseNotes/21.rst

@@ -76,6 +76,9 @@ Improvements and New Features
 - The ``bitset::to_string`` function has been optimized, resulting in a performance improvement of up to 8.3x for bitsets
   with uniformly distributed zeros and ones, and up to 13.5x and 16.1x for sparse and dense bitsets, respectively.
 
+- The ``flat_map::insert`` and ``flat_set::insert_range`` have been optimized, resulting in a performance improvement of up
+  to 10x for inserting elements into a ``flat_map`` when the input range is a ``flat_map`` or a ``zip_view``.
+
 Deprecations and Removals
 -------------------------
 

libcxx/test/libcxx/iterators/product_iterator.pass.cpp

@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
+// gcc 15 does not seem to recognize the __product_iterator_traits specializations
+// UNSUPPORTED: gcc
 
 #include <flat_map>
 #include <ranges>