fix: Updates dependencies & local development environment

[timnolte]

All Submissions:

• Have you followed the plugin Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests) for the same update/change?

Changes proposed in this Pull Request:

• Updates Composer dependencies to latest versions with support for PHP 8.0.
• Updates NPM dependencies to latest versions with support for Node 16.
• Removes wp-env and migrates to a Dev Container for local development.
• Updates linting configuration.
• Updates unit tests to pass linting testings.
• Updates unit testing to use new Dev Container & Docker setup.
• Fixes linting issues.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?