Bump wordpress versions

[jeffpaul]

Description of the Change

Bump WP tested-up-to 7.0 and minimum to 6.8.

Closes #267.

Copilot summary

This pull request updates compatibility and requirements for the plugin to support newer WordPress versions. The most important changes are:

WordPress Version Support:

• Updated the minimum required WordPress version from 6.6 to 6.8 in the plugin header (simple-page-ordering.php).
• Updated the test matrix in the GitHub Actions workflow to use WordPress 6.8 as the minimum version instead of 6.6 (.github/workflows/cypress.yml).

Documentation:

• Updated the "Tested up to" value in readme.txt to indicate compatibility with WordPress 7.0.

How to test the Change

Changelog Entry

Changed - Bump WordPress tested-up-to version 7.0.
Changed - Bump WordPress minimum supported version to 6.8.

Credits

Props @jasperfrontend, @jeffpaul.

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.