[4.2.x] Refs #30220 -- Bumped required version of Selenium to 3.8.0.

Follow up to 8d010f3986.
Backport of 06881341d4 from main

docs/topics/testing/tools.txt

@@ -1035,7 +1035,7 @@ First of all, you need to install the :pypi:`selenium` package:
 
 .. console::
 
-    $ python -m pip install selenium
+    $ python -m pip install "selenium >= 3.8.0"
 
 Then, add a ``LiveServerTestCase``-based test to your app's tests module
 (for example: ``myapp/tests.py``). For this example, we'll assume you're using