chore: prepare for laravel 11

Description

Adds support for Laravel 11 and drops support for Laravel 9, as it is end-of-life. Therefore, the minimum PHP-version has been bumped to 8.1. Tests are now running on PHPunit 10 and the versions of the Github-actions are updated to the latest stable versions.

Motivation and context

Laravel 11 will be released in the next few weeks.

How has this been tested?

All tests are running fine with the new versions, tests will run for PHP 8.3 as well.

Screenshots (if appropriate)

Types of changes

Strictly, it's a breaking change because it's dropping support for Laravel 9 and PHP 8.0.

What types of changes does your code introduce? Put an x in all the boxes that apply:

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

Please, please, please, don't send your pull request until all of the boxes are ticked. Once your pull request is created, it will trigger a build on our continuous integration server to make sure your tests and code style pass.

[x] I have read the CONTRIBUTING document.
[x] My pull request addresses exactly one patch/feature.
[x] I have created a branch for this patch/feature.
[x] Each individual commit in the pull request is meaningful.
[x] I have added tests to cover my changes.
[x] If my change requires a change to the documentation, I have updated it accordingly.

If you're unsure about any of these, don't hesitate to ask. We're here to help!

craigpaul / laravel-postmark