Remove duplicate words from docs (#6398)

Remove the duplicate usage of “required” in documentation (specifically, `introduction.rst`).

docs/getting-started/introduction.rst

@@ -45,11 +45,12 @@ What do I need?
     - PyPy ❨5.4, 5.5❩
 
     This is the last version to support Python 2.7,
-    and from the next version (Celery 5.x) Python 3.5 or newer is required.
+    and from the next version (Celery 5.x) Python 3.5 or newer.
 
     If you're running an older version of Python, you need to be running
     an older version of Celery:
 
+    - Python 2.7 or Python 3.5: Celery series 4.4 or earlier.
     - Python 2.6: Celery series 3.1 or earlier.
     - Python 2.5: Celery series 3.0 or earlier.
     - Python 2.4 was Celery series 2.2 or earlier.