GitHub Actions workflow to validate go.mod, go.sum, and vendor directory

[zrhoffman]

What does this PR (Pull Request) do?

As a follow-up to #5229, this PR

• Adds a GitHub Actions workflow to verify that go.mod, go.sum, and the vendor directory have no uncommitted Git changes when modules are vendored.
• Removes an unused dependency and updates go.sum
• Gets go mod vendor to run successfully in the CiaB Traffic Ops integration service's Dockerfile by including the v4-client directory. Error from go mod vendor that is fixed:

  #29 28.09 github.com/apache/trafficcontrol/traffic_ops/testing/api/v4 tested by
  #29 28.09       github.com/apache/trafficcontrol/traffic_ops/testing/api/v4.test imports
  #29 28.09       github.com/apache/trafficcontrol/traffic_ops/v4-client: no matching versions for query "latest"

• Gets the Dockerized TO and TM unit tests to working again by including go.mod/go.sum in the Docker image.
• is not related to any Issue

Which Traffic Control components are affected by this PR?

• CDN in a Box
• CI tests

What is the best way to verify this PR?

• Verify the GitHub Action is working as intended.

  • Check the result of the Check Go modules workflow at 0e54b99. The following steps are expected to have failed:

    • Untracked changes to vendored dependencies
    • Changes to go.sum

  • Check the result of the Check Go modules workflow at fb9f772. It is expected to succeed.

• Run the TO API tests in CDN in a Box. On my computer, it took 55 minutes from the time when the API tests started running.

  docker-compose -f docker-compose.yml -f docker-compose.traffic-ops-test.yml up --no-deps -d edge dns db smtp trafficops-perl trafficops trafficvault integration;
  docker-compose -f docker-compose.traffic-ops-test.yml logs -f integration;

• Run the Traffic Monitor unit tests in Docker

  docker-compose -f traffic_monitor/tests/docker-compose.yml up --build

• Run the Traffic Ops unit tests in Docker

  docker-compose -f traffic_ops/app/bin/tests/docker-compose.yml up --build

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

• master (09e2501)

The following criteria are ALL met by this PR

• This PR includes tests
• This PR includes documentation
• An update to CHANGELOG.md is not necessary
• 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)

[ocket8888]

LGTM - also happy to get rid of some more references to the un-versioned client