Add standard 5XX alert for load balancer
## Description
Add a standard alert which is triggered by a set number of `5XX` status codes from the load balancer being seen over a period of a minute.
## Further details
This alert should:
- Be on by default
- Have configuration for:
- Turning the alert on or off (default: on)
- Time period to aggregate over (default: 1 minute)
- Threshold for alert to trigger (default: 10)
- The alert policy & condition names
- Default: see https://gitlab.developers.cam.ac.uk/uis/devops/iam/authentication/infrastructure/-/issues/191
- Be present in the `v9` and `v8` branches, i.e. **this change will require two merge requests**
## Task list
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## Acceptance criteria
- [ ] Cloud run webapp terraform module includes standard alert for 5XX responses
- [ ] Configuration is as described above - in particular this alert should be specifically configured on or off by itself (i.e. not tied to all alerts being enabled or not)
## Links/references
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issue