We now need at least Flask-WTF 0.15 (#24621)

We upgraded flask and werkzeug in #24399, and updated the constraints,
but not everyone uses them (such as me in my local virtual environment
when developing) so the min version in setup.cfg has to match as well

setup.cfg

@@ -111,7 +111,7 @@ install_requires =
     flask-caching>=1.5.0
     flask-login>=0.5
     flask-session>=0.4.0
-    flask-wtf>=0.14.3
+    flask-wtf>=0.15
     graphviz>=0.12
     gunicorn>=20.1.0
     httpx