chore(swift): update renovate to handle swift-version file

Author: Fluf22

renovate.json

@@ -173,6 +173,15 @@
       "datasourceTemplate": "github-tags",
       "depNameTemplate": "php/php-src"
     },
+    {
+      "description": "Update .swift-version",
+      "fileMatch": ".swift-version",
+      "matchStrings": [
+        "(?<currentValue>.*)"
+      ],
+      "datasourceTemplate": "swift",
+      "depNameTemplate": "Swift"
+    },
     {
       "description": "Update swiftformat version",
       "fileMatch": [