Update minimum sphinx versions after upgrading sphinx-autoapi (#20170)

* Allow point releases of AutoAPI 1.8 (I used with 1.8.4 in all my testing)
* Require at least Sphinx v4

  A few things got deprecated in Sphinx 4, and as this dep is only for
  us building docs we can pick and choose what we like without impacting
  users, so lets stay up-to-date.

setup.py

@@ -253,11 +253,10 @@ def write_version(filename: str = os.path.join(*[my_dir, "airflow", "git_version
 ]
 doc = [
     'click>=7.1,<9',
-    # Sphinx is limited to < 3.5.0 because of https://github.com/sphinx-doc/sphinx/issues/8880
-    'sphinx>=3.5.0, <5.0.0',
+    'sphinx>=4.0.0, <5.0.0',
     'sphinx-airflow-theme',
     'sphinx-argparse>=0.1.13',
-    'sphinx-autoapi==1.8.0',
+    'sphinx-autoapi~=1.8.0',
     'sphinx-copybutton',
     'sphinx-jinja~=1.1',
     'sphinx-rtd-theme>=0.1.6',