OpenAPI 3.1 support

[osteel]

Description

This PR bumps the version of OpenAPI PSR7 Validator for OpenAPI 3.1 support.

It also upgrades PHPUnit to v9.0+ and drops support for PHP 7.2.

Motivation and context

OpenAPI 3.1 was released 2 years ago, but support for it was blocked by an underlying package that seems pretty much abandoned.

The community stepped up and forked it to finally provide support for OpenAPI 3.1, which was introduced with this PR.

How has this been tested?

Test suite.

Types of changes

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)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

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