2025 eol cleanup

[luispabon]

This PR removes a lot of legacy stuff, and revises documentation.

CI/CD & Build Pipeline

• Simplify Build Matrix: Refactored the GitHub Actions workflow to use a dynamic matrix for PHP versions (8.1, 8.2, 8.3, 8.4) instead of explicit configuration blocks, significantly reducing configuration boilerplate.
• Update Actions: Upgraded actions/checkout to v6.
• Remove Tertiary Builds: Removed the "tertiary" container build step (previously used for Swoole variants) from the pipeline.

Deprecations & Removals

• Drop PHP 7.4 & 8.0: Removed build support and configuration files for PHP 7.4 and 8.0 as their base image (Ubuntu 20.04) has reached End-of-Life.
• Clean up Files: Deleted Dockerfiles, READMEs, and configuration overrides associated with the removed PHP versions (php/7.4/).

Documentation

• Revise README.md to clean up references to old stuff that doesn't apply anymore and for clarity.
• Update Support Status: Updated README.md to explicitly mark PHP 7.4 and 8.0 as unsupported/EOL.