fix(release-please): tagPullRequestNumber as branch's properties schema (#5688)

This fixes the config schema error:

```
[
{
"instancePath": "/branches/0",
"schemaPath": "#/allOf/1/properties/branches/items/additionalProperties",
"keyword": "additionalProperties",
"params": {
"additionalProperty": "tagPullRequestNumber"
},
"message": "must NOT have additional properties"
}
]
```

https://github.com/googleapis/google-cloud-go/pull/11753/checks?check_run_id=38345091141

Author: suztomo

packages/release-please/src/config-schema.json

@@ -201,6 +201,7 @@
                             "releaseType": true,
                             "packageName": true,
                             "handleGHRelease": true,
+                            "tagPullRequestNumber": true,
                             "bumpMinorPreMajor": true,
                             "bumpPatchForMinorPreMajor": true,
                             "path": true,