ci: drop travis ci and appveyor in favor of GitHub actions

[ChristianMurphy]

Checklist

• the individual contributor license agreement is signed
• commit message follows commit guidelines
• tests are included
• documentation is changed or added
• message properties have been updated with new phrases
• view conforms with WCAG 2.0 AA

Description of change

travis ci updated their pricing model to cap open source projects.
https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

GitHub actions is another free for open source continuous integration platform, which has more generous terms.
https://docs.github.com/en/free-pro-team@latest/actions/reference/usage-limits-billing-and-administration

AppVeyor was being used exclusively for Windows CI testing, GitHub actions can test on Linux, MacOS, and Windows.
To simplify the CI process, dropping AppVeyor as well, and moving forward with GitHub actions as the sole CI provider.

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GitHub actions was originally configured in #2217 and #2218

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the documentation has been updated to reflect these change, replacing existing CI badges for uP 5 with the new GitHub actions badge

[ChristianMurphy]

@bjagg would you be able to remove to webhook for AppVeyor from the repo, so we don't trigger accidentally trigger AppVeyor builds?
The hook should be listed and removable at https://github.com/Jasig/uPortal/settings/hooks

[cbeach47]

@bjagg removed the Appveyor and Travis CI webhooks from the uPortal repo settings. I'll merge this PR.