Update php version in README.

README.md

@@ -1,6 +1,6 @@
 # ![Islandora](https://cloud.githubusercontent.com/assets/2371345/25624809/f95b0972-2f30-11e7-8992-a8f135402cdc.png) Islandora
 
-[![Minimum PHP Version](https://img.shields.io/badge/php-%3E%3D%207.2-8892BF.svg?style=flat-square)](https://php.net/)
+[![Minimum PHP Version](https://img.shields.io/badge/php-%3E%3D%207.4-8892BF.svg?style=flat-square)](https://php.net/)
 [![Build Status](https://github.com/islandora/islandora/actions/workflows/build-2.x.yml/badge.svg)](https://github.com/Islandora/islandora/actions)
 [![Contribution Guidelines](http://img.shields.io/badge/CONTRIBUTING-Guidelines-blue.svg)](./CONTRIBUTING.md)
 [![LICENSE](https://img.shields.io/badge/license-GPLv2-blue.svg?style=flat-square)](./LICENSE)