* chore: migrate to GitHub Actions for CI

- Use headless browsers instead of xfvb where possible.
- Replace commitlint-travis with usage of the base commitlint as there is no official CLI for GitHub Actions.
- Remove CI badges as GitHub displays the CI status natively for each commit.

* chore: use BS plugin's compatible env names

See: https://github.com/karma-runner/karma-browserstack-launcher#:~:text=username%20your%20BS,BROWSERSTACK_ACCESS_KEY%20env%20variable.

* chore: improve GH Actions workflow's job names

This is needed because in GH's configuration, we need to distinguish the required checks for protected branches and giving them unique names results in easiera configs.

* chore: remove client testing from the windows job of test.yml

The purpose of this test is to make sure that client JS works in different browsers and it does not need to be run on different OS/Node versions.

Co-authored-by: Jonathan Ginsburg <[email protected]>

This is the [default environment value name provided by GitHub Actions][1].

Also:

* Format the YAML workflow files.

[1]: https://docs.github.com/en/actions/security-guides/automatic-token-authentication#about-the-github_token-secret

In our client tests, for BrowserStack, reduce the `browserDisconnectTimeout`
and the `browserNoActivityTimeout` to a minute, instead of 5. Also, reduce the
concurrency of browsers to 2 instead of 3. This is because BS is being flaky
when handling many browsers concurrently and, when a browser has been
disconnected it will generally not come back, so we do not need to wait a whole
5 minutes; it is better to just use another try from the
`browserDisconnectTolerance` configuration pool. The overall result is a more
efficient CI run.

We have a script, which is run as part of release process to automatically
update the documentation website:
https://github.com/karma-runner/karma-runner.github.com. The DOCS_GITHUB_TOKEN
secret contains a GitHub token with write access to the website repository.

This should've happened in
4b3b072.

## [6.3.5](v6.3.4...v6.3.5) (2021-10-20)

### Bug Fixes

* **client:** prevent socket.io from hanging due to mocked clocks ([#3695](#3695)) ([105da90](105da90))