Test on PHP 8.1

[GrahamCampbell]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	-
Documentation	-
License	MIT

What's in this PR?

Tests on PHP 8.1.

Why?

So that developers can use this package with confidence on PHP 8.1.

Example Usage

// If you added new features, show examples of how to use them here
// (remove this section if not a new feature)

$foo = new Foo();

// Now we can do
$foo->doSomething();

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

To Do

• If the PR is not complete but you want to discuss the approach, list what remains to be done here

[GrahamCampbell]

PHPStan failures are not related.

[dbu]

thanks a lot!