Configure renovate to update Go version in go.mod (#18)

Author: dmjb

go.mod

@@ -1,6 +1,6 @@
 module github.com/stacklok/toolhive-core
 
-go 1.25.6
+go 1.25.7
 
 require (
 	github.com/google/cel-go v0.27.0

renovate.json

@@ -6,5 +6,15 @@
     ":semanticCommitsDisabled"
   ],
   "labels": ["dependencies"],
-  "postUpdateOptions": ["gomodTidy"]
+  "postUpdateOptions": ["gomodTidy"],
+  "customManagers": [
+    {
+      "customType": "regex",
+      "fileMatch": ["(^|/)go\\.mod$"],
+      "matchStrings": ["go (?<currentValue>\\d+\\.\\d+(\\.\\d+)?)"],
+      "depNameTemplate": "golang",
+      "datasourceTemplate": "golang-version",
+      "versioningTemplate": "loose"
+    }
+  ]
 }