Refactor CI workflows and standardize PHP version to 8.0

[dannorthern]

This pull request streamlines and modernizes the repository's GitHub Actions workflows by migrating custom workflow logic to reusable workflows from the organization's shared .github repository. It also slightly broadens PHP version compatibility in composer.json. The main changes are summarized below:

CI/CD Workflow Modernization:

• The release.yml workflow is refactored to use the shared composer-package-release.yml reusable workflow. This removes a large amount of custom scripting in favor of a standardized, maintainable release process.
• The tests.yml workflow is replaced with a call to the shared composer-package-ci.yml reusable workflow, simplifying the CI configuration and ensuring consistency with other packages.
• A new scheduled-ci.yml workflow is added to run scheduled CI checks every Monday using the shared workflow, improving ongoing code health monitoring.

Dependency Requirements:

• The minimum required PHP version in composer.json is lowered from 8.1 to 8.0, allowing broader compatibility.