C++, bump env version due to AST and Symbol changes

sphinx/domains/cpp.py

@@ -8149,7 +8149,7 @@ def setup(app: Sphinx) -> Dict[str, Any]:
 
     return {
         'version': 'builtin',
-        'env_version': 7,
+        'env_version': 8,
         'parallel_read_safe': True,
         'parallel_write_safe': True,
     }