Revert "[libc++] Require C++20 to build the benchmarks."

There are build bots without C++20 support building the benchmarks.

This reverts commit 34acc91642440b8e4bad17acfdbb1314c8f2043e.

GitOrigin-RevId: 171956aab32efa2d2f4999335a3acec15a9472d9
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index d9302fe..00ada48 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -43,5 +43,4 @@
 API Changes
 -----------
 
-- Starting with libc++13 a C++20 capable compiler is required to build the
-  libc++ benchmarks.
+- ...