Change PHP default to 8.3.x (required for cflinuxfs5 packaging)

PHP 8.1.x is EOL (2024-11-25) and is not available on cflinuxfs5.
buildpack-packager validates that the default version exists for every
target stack, so packaging for cflinuxfs5 failed with:

  No matching default dependency `php` for stack `cflinuxfs5`

Change the default to 8.3.x which is present in both cflinuxfs4 and
cflinuxfs5. This also aligns the default with the actively maintained
PHP stream.

Author: ivanovac

manifest.yml

@@ -2,7 +2,7 @@
 language: php
 default_versions:
 - name: php
-  version: 8.1.32
+  version: 8.3.x
 - name: httpd
   version: 2.4.66
 - name: nginx