ci: update dotnet version with renovate

Signed-off-by: Alexander Linne <[email protected]>

renovate.json

@@ -1,12 +1,26 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": ["config:base"],
+  "customManagers": [
+    {
+      "customType": "regex",
+      "fileMatch": ["^\\.github/workflows/.+?\\.yaml$"],
+      "matchStrings": ["dotnet-version: (?<currentValue>.+?)\\s"],
+      "datasourceTemplate": "dotnet-version",
+      "depNameTemplate": "dotnet-sdk",
+      "depTypeTemplate": "action"
+    }
+  ],
   "packageRules": [
     {
       "matchManagers": ["nuget"],
       "matchPackagePrefixes": ["xunit", "nunit", "NUnit", "MSTest"],
       "matchPackageNames": ["Microsoft.NET.Test.Sdk", "JetBrains.Annotations"],
       "enabled": false
+    },
+    {
+      "matchDatasources": ["dotnet-version"],
+      "groupName": "dotnet version"
     }
   ]
 }