[ARI/Buildpacks/PHP] add @arjun024 as php subteam approver

I have formerly maintained the PHP buildpack and interested to be an approver.

Please see my contributions https://github.com/cloudfoundry/php-buildpack/commits?author=arjun024

Author: arjun024

toc/working-groups/app-runtime-interfaces.md

@@ -156,6 +156,8 @@ areas:
     github: thitch97
   - name: Sophie Wigmore
     github: sophiewigmore
+  - name: Arjun Sreedharan
+    github: arjun024
   repositories:
   - cloudfoundry/php-buildpack
   - cloudfoundry/php-buildpack-release