chore: roll back .NET to 8, use standard renovate versioning

* Restored .NET to v8
* Removed "loose" renovate versioning option to prevent updating to
  preview versions across all dependencies
* Removed allowedVersion rules preventing preview versions for
  specific dependencies, which are now unnecessary

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "9.0.100-rc.1.24452.12",
+    "version": "8.0.402",
     "rollForward": "latestMinor"
   },
   "msbuild-sdks": {

renovate.json

@@ -5,22 +5,19 @@
     ":semanticCommits"
   ],
   "prHourlyLimit": 2,
-  "versioning": "loose",
   "packageRules": [
     {
       "matchPackagePatterns": [
         "*"
       ],
       "groupName": "all dependencies",
       "groupSlug": "all-deps",
-      "automerge": true,
-      "allowedVersions": "!/preview/"
+      "automerge": true
     },
     {
       "matchPackagePrefixes": [
         "dotnet-sdk"
-      ],
-      "allowedVersions": "!/preview/"
+      ]
     },
     {
       "matchPackagePrefixes": [