Use ATS 8.1 in CDN in a Box

[zrhoffman]

What does this PR (Pull Request) do?

• This PR fixes ATS in CiaB: /usr/bin/traffic_cop: error while loading shared libraries: libhwloc.so.5 #5918 by using a Traffic Server RPM built for the latest version of CentOS 8 and getting the CDN-in-a-Box CI GitHub Actions workflow to pass.
• This PR fixes CDN in a Box should use ATS 8.1 #5919 by making CDN in a Box use a trafficserver RPM built from the 8.1.x ATS release branch.

Which Traffic Control components are affected by this PR?

• CDN in a Box
• Documentation
• GitHub Actions
• T3C integration tests

What is the best way to verify this PR?

1. Build and run CDN in a Box, ensure caches are reported as healthy on http://trafficmonitor.infra.ciab.test/

   make -j6
   docker-compose build --parallel
   docker-compose kill && docker-compose down -v && docker-compose up -d
   docker-compose logs -f trafficmonitor

2. Build and run CDN in a Box for CentOS 7, ensure caches are reported as healthy on http://trafficmonitor.infra.ciab.test/

   export RHEL_VERSION=7
   make -j6
   docker-compose build --parallel
   docker-compose kill && docker-compose down -v && docker-compose up -d
   docker-compose logs -f trafficmonitor

3. Check the result of the CDN-in-a-Box CI GitHub Actions workflow

If this is a bug fix, what versions of Traffic Control are affected?

• master (5ad059f)
• 5.1.2

The following criteria are ALL met by this PR

• This PR includes tests
• This PR includes documentation
• This PR includes an update to CHANGELOG.md
• This PR includes any and all required license headers
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)