PHP 8.1 support

[chrisnharvey]

Describe your change PHP 8.1 support added

Motivation and Context PHP 8.1 has been available for a while now.

Type of Change 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).
• [x] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).

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

• [x] I've read the Contribution Guide.
• [x] I've updated the documentation.
• [x] I've targeted the correct branch with this Pull Request.