This commit:
* Adds a GH Actions workflow for the CI checks which were previously run on Travis in the `test` and `coverage` stages.
* Removes the, now redundant, `.travis.yml` configuration.
* Updates the `.gitattributes` file.
* Updates the "Build Status" badge in the Readme to use the results from the GH `Test` Actions runs.
* Updates the "Tested on" badge in the Readme to include PHP 8.
* Updates a test file which mentioned Travis.

Notes:
1. Each of these jobs has a `needs` dependency on the previous job to prevent it from starting if the previous job failed.
    While not 100% necessary, this is just an efficiency tweak, being kind to the free service being offered as we know that if linting fails, the tests will fail etc.
2. The builds in the `test` and `coverage` jobs are essentially the same as previously run on Travis, though PHP 8.0 is now fully accounted for and `8.1` is set as `nightly`.
3. Previously, this "stage" would run on al...