ci: disable renovate updates for placeholder versions

Explicitly disable Renovate updates targeting the `0.0.0-PLACEHOLDER` and `0.0.0-EXPERIMENTAL-PLACEHOLDER` versions. These placeholders are not intended for automated updates and require manual handling.

Author: alan-agius4

renovate.json

@@ -37,6 +37,14 @@
       "matchPackageNames": ["quicktype-core"],
       "schedule": ["before 4:00am on the first day of the month"]
     },
+    {
+      "matchCurrentVersion": "0.0.0-PLACEHOLDER",
+      "enabled": false
+    },
+    {
+      "matchCurrentVersion": "0.0.0-EXPERIMENTAL-PLACEHOLDER",
+      "enabled": false
+    },
     {
       "groupName": "angular",
       "matchDepNames": ["/^@angular/.*/", "/angular/dev-infra/"]