Warn about falling back to jinja2_native=false (#49063)

changelogs/fragments/jinja2_native-fallback-warning.yaml

@@ -0,0 +1,2 @@
+minor_changes:
+  - Add warning about falling back to jinja2_native=false when Jinja2 version is lower than 2.10.

lib/ansible/template/__init__.py

@@ -74,6 +74,11 @@ if C.DEFAULT_JINJA2_NATIVE:
     except ImportError:
         from jinja2 import Environment
         from jinja2.utils import concat as j2_concat
+        from jinja2 import __version__ as j2_version
+        display.warning(
+            'jinja2_native requires Jinja 2.10 and above. '
+            'Version detected: %s. Falling back to default.' % j2_version
+        )
 else:
     from jinja2 import Environment
     from jinja2.utils import concat as j2_concat