feat: attempt to add osv SCHEMA_VERSION to renovate (google#4973)

Trying to fix google#3816
I'm not 100% sure on the syntax of this configuration, and I can't work
out how to test this locally.
Let's just try this

Author: michaelkedar

renovate.json

@@ -169,7 +169,8 @@
     },
     {
       "matchDatasources": [
-        "git-refs"
+        "git-refs",
+        "github-tags"
       ],
       "groupName": "submodules"
     },
@@ -218,6 +219,18 @@
       "packageNameTemplate": "https://github.com/ossf/osv-schema",
       "datasourceTemplate": "git-refs",
       "versioningTemplate": "git"
+    },
+    {
+      "customType": "regex",
+      "managerFilePatterns": [
+        "osv/models.py"
+      ],
+      "matchStrings": [
+        "SCHEMA_VERSION\\s*=\\s*'(?<currentValue>.*)'"
+      ],
+      "packageNameTemplate": "ossf/osv-schema",
+      "datasourceTemplate": "github-tags",
+      "versioningTemplate": "semver"
     }
   ],
   "ignorePaths": ["docker/poetry/requirements.txt"]