Update php.yml

Bump to php 7.4 as min version as required by openapi-generator.

.github/workflows/php.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-versions: ['7.2', '7.3', '7.4', '8.0']
+        php-versions: ['7.4', '8.0']
     steps:
       - uses: actions/checkout@v2