Set Twig 2 end of maintenance to December 2023

CHANGELOG

@@ -1,3 +1,5 @@
+**End of maintainance of version 2 is scheduled for end of December 2023**
+
 # 2.15.6 (2023-XX-XX)
 
  * n/a

README.rst

@@ -6,6 +6,9 @@ Twig is a template language for PHP.
 Twig uses a syntax similar to the Django and Jinja template languages which
 inspired the Twig runtime environment.
 
+**Twig version 2 end of maintainance is scheduled for end of December 2023.**
+Please, upgrade at your earliest convenience.
+
 Sponsors
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