chore(deps): bump PHP requirement to ^8.3 (#6)

Brings the composer require.php constraint in line with the runtime
the container + CI already use (PHP 8.3). Drops formal support for
PHP 8.1 and 8.2. Several vendor packages (e.g. azjezz/psl@4.3.0) use
PHP 8.3 typed class constants, making the codebase functionally
8.3-only already.

Part of the fleet-wide PHP 8.3 sweep.

Author: rubenvdlinde

composer.json

@@ -56,7 +56,7 @@
 		]
 	},
 	"require": {
-		"php": "^8.1"
+		"php": "^8.3"
 	},
 	"require-dev": {
 		"nextcloud/coding-standard": "^1.4",