Update the version of Sphinx in our CI machines

[bneradt]

Currently, the CI builds our ATS Sphinx docs with Sphinx version 2.0.1. This can be seen from this job:
https://ci.trafficserver.apache.org/view/10.x%20master/job/docs-master/

Making html in uml
...
Running Sphinx v2.0.1
...

Currently the latest version is 4.0.2. We are currently pinned to the 3.x releases however due to the issue described in #7941.

The fix for that is slated for the next Sphinx 4.0.3 release (see sphinx-doc/sphinx#9322).

Thus, currently, developers will build their docs with Sphinx 3.x on their local machines, and then CI will run in v2.0.1. This difference can cause unexpected Sphinx compilation issues. We should update the CI machine with a more recent Sphinx version.

Before we do this, however, we need to make sure that the docs will build for our supported 8.x, 9.x, and master branches using the later Sphinx version.

[bneradt]

Before we can update CI, we need to update our 8.1.x and 7.1.x branches for Sphinx 3.x. Currently building docs on those branches with Sphinx 3.x fails due to a small variety of issues. These PRs unblock this issue:

• 8.1.x doc tweaks to support Sphinx 3.x #7984
• 7.1.x doc tweaks to support Sphinx 3.x #7985

[bneradt]

In addition to the above, I noticed 9.0.x needs a couple updates as well for Sphinx 3.x:

• Pin Sphinx to 3.x to unblock make html #7987

[bneradt]

Resolving this issue (i.e., updating CI to have Sphinx 3.x installed) unblocks #7978

[bneradt]

This should be fixed now with the new CI.