Fix a handful of minor bugs within the Semgrep Merge Tool
- Bump down symfony/yaml version from 6.x to 5.x to support php 7.x on toolforge
- Rebuild composer.lock
- Rename src/YAMLManager.php to src/YamlManager.php
- Update templates/index.twig.html to use update README link
- Remove extraneous comma in function call - php 7.x doesn't like this
- Pass explicit $cache_path argument to function
Bug: T312807
Edited by SBassett