Update Traffic Router Java version to 11

[zrhoffman]

Closes: #3196

This PR makes Traffic Router use Java 11 by default for compilation, testing, and running.

Big thanks to @ARMmaster17 for getting Traffic Router itself to work using Java 11. Besides changing Java versions around the project, this PR mainly just updates PMD to a version that supports targeting Java 11, which involved fixing new PMD "failures" that PMD 6.29.0 (up from PMD 5.3.5 in maven-pmd-plugin version 3.6) finds.

---

Which Traffic Control components are affected by this PR?

• Documentation
• Traffic Router
• Automation (GitHub Actions, Ansible Roles)

What is the best way to verify this PR?

• Run tests using Java 11

  docker-compose -f traffic_router/tests/docker-compose.yml up --build unit_router

• Compile Traffic Router using Java 11:

  ./pkg -bv traffic_router_build

• Verify that Traffic Router, including the resource watchers, work as expected by running the TR external and integration tests

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[rawlinp]

I'm getting some interesting differences in performance with testing this PR. Specifically, TR with this PR appears to handle about 3365 req/s in my shadow testing environment, whereas master handles 4549 req/s when compiled and run using Java 8 and 4959 req/s when compiled using Java 8 but run with Java 11.

So, this PR appears to decrease performance by 25% in terms of req/s handled compared to master on Java 8. If we just upgrade to java 11 on the server, we get a performance increase of 9%.

[zrhoffman]

Rebased to resolve merge conflicts

        both modified:   CHANGELOG.md
        deleted by us:   docs/source/admin/traffic_router/migrationto2-3.rst

[zrhoffman]

Rebased to fix a merge conflict from #6278

[zrhoffman]

Rebased to fix a merge conflict from #6303

[zrhoffman]

So, this PR appears to decrease performance by 25% in terms of req/s handled compared to master on Java 8. If we just upgrade to java 11 on the server, we get a performance increase of 9%.

There was a NullPointerException due to trying to get the socket from socketWrapper when it was null, which decreased performance significantly. Fixed in 35bd577, performance is back up for me