C++, update env version due to recent changes

Author: jakobandersen

sphinx/domains/cpp.py

@@ -7895,7 +7895,7 @@ def initStuff(app):
 
     return {
         'version': 'builtin',
-        'env_version': 3,
+        'env_version': 4,
         'parallel_read_safe': True,
         'parallel_write_safe': True,
     }