Upgrade to SilverStripe 6 (#17)

- Update PHP requirement to ^8.3
- Update dnadesign/silverstripe-elemental: ^5 → ^6
- Update silverstripe/framework: ^5 → ^6
- Update silverstripe/recipe-testing: ^3 → ^4
- No code changes required (no SS6 breaking changes)

Author: jsirish

composer.json

@@ -18,11 +18,12 @@
         }
     ],
     "require": {
-        "dnadesign/silverstripe-elemental": "^5.0",
-        "silverstripe/framework": "^5"
+        "php": "^8.3",
+        "dnadesign/silverstripe-elemental": "^6.0",
+        "silverstripe/framework": "^6"
     },
     "require-dev": {
-        "silverstripe/recipe-testing": "^3"
+        "silverstripe/recipe-testing": "^4"
     },
     "minimum-stability": "dev",
     "prefer-stable": true,