Cannot assign to read only property 'schemaInsightType'

**Describe the problem**

A type error was encountered in the insights page.

**To Reproduce**

The replay isn't available but the error happened on the `cluster/insights/` route with the following error:

```
TypeError: Cannot assign to read only property 'schemaInsightType' of object '#'
at ie @ https://cockroachlabs.cloud/assets/admin-ui-22-2.5622f6dbabb9cf5ce2d2.js:1496:11262
at onClick @ https://cockroachlabs.cloud/assets/admin-ui-22-2.5622f6dbabb9cf5ce2d2.js:1496:11587
at Object.ni @ https://cockroachlabs.cloud/assets/common.e8e4c759e5a2823aaa26.js:1037:9416
at jt @ https://cockroachlabs.cloud/assets/common.e8e4c759e5a2823aaa26.js:1037:9579
at un @ https://cockroachlabs.cloud/assets/common.e8e4c759e5a2823aaa26.js:1037:9642
at mi @ https://cockroachlabs.cloud/assets/common.e8e4c759e5a2823aaa26.js:1037:29163
at Ii @ https://cockroachlabs.cloud/assets/common.e8e4c759e5a2823aaa26.js:1037:29624
at @ https://cockroachlabs.cloud/assets/common.e8e4c759e5a2823aaa26.js:1037:35032
at Dt @ https://cockroachlabs.cloud/assets/common.e8e4c759e5a2823aaa26.js:1041:19094
at Nr @ https://cockroachlabs.cloud/assets/common.e8e4c759e5a2823aaa26.js:1037:8567
```
Based on this and the prettified stack trace, it seems to have been originating from the `getSchedules` function in `schedulesApi.ts`.

**Additional data / screenshots**
- [Datadog error](https://cockroachlabs-prod.datadoghq.com/rum/explorer?query=%40view.name%3A%28%22%2Fcluster%2F%3Acluster%2Fdatabases%2F%3Adb%22%20OR%20%22%2Fcluster%2F%3Acluster%2Finsights%22%20OR%20%22%2Fcluster%2F%3Acluster%2Fsql-activity%22%20OR%20%22%2Fcluster%2F%3Acluster%2Fdatabases%2F%3Adb%2F%3Aschema%2F%3Atable%22%20OR%20%22%2Fcluster%2F%3Acluster%2Fdatabases%22%29%20-%40error.source%3Anetwork%20version%3A2023-05-10-0%20%40type%3Aerror&cols=&event=AgAAAYgLunL4CdTYTQAAAAAAAAAYAAAAAEFZZ0x1bmgyQUFDeXdtb3J6S25NZmdBRgAAACQAAAAAMDE4ODBjNjQtMmQ5My00ZGIzLTgwNDgtZjcxNzA4MjU3OWM3&index=%2A&p_tab=error_details&type=rum&viz=stream&from_ts=1683820800000&to_ts=1683828000000&live=false)

**Environment:**

CockroachDB version: Cockroach Serverless
Server OS: Mac OS X 10.15.7

Jira issue: CRDB-27932