Update ghcr.io/twin/gatus Docker tag to v5.21.0 (#1105)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [ghcr.io/twin/gatus](https://github.com/TwiN/gatus) | minor | `v5.20.0` -> `v5.21.0` |

---

### Release Notes

<details>
<summary>TwiN/gatus (ghcr.io/twin/gatus)</summary>

### [`v5.21.0`](https://github.com/TwiN/gatus/releases/tag/v5.21.0)

[Compare Source](https://github.com/TwiN/gatus/compare/v5.20.0...v5.21.0)

#### What's Changed

- feat(external-endpoint): Implement heartbeat by [@&#8203;TwiN](https://github.com/TwiN) in https://github.com/TwiN/gatus/pull/1173
- feat(alerting): Support sending messages to Telegram topics in group by [@&#8203;medaved](https://github.com/medaved) in https://github.com/TwiN/gatus/pull/928
- feat: Add body to TCP, UDP, and TLS endpoints and templating by [@&#8203;jasonshugart](https://github.com/jasonshugart) in https://github.com/TwiN/gatus/pull/1134
- fix(websocket): add support for custom headers in WS check by [@&#8203;olexandrd](https://github.com/olexandrd) in https://github.com/TwiN/gatus/pull/1180
- fix(websocket): use tls config by [@&#8203;joy4eg](https://github.com/joy4eg) in https://github.com/TwiN/gatus/pull/1165
- docs: Update example Grafana dashboard by [@&#8203;perfectra1n](https://github.com/perfectra1n) in https://github.com/TwiN/gatus/pull/1163
- chore(deps): bump google.golang.org/api from 0.236.0 to 0.242.0 by [@&#8203;dependabot](https://github.com/dependabot)\[bot] in https://github.com/TwiN/gatus/pull/1164
- chore(deps): Bump TwiN/whois dependency to v1.1.11 by [@&#8203;Byh0ki](https://github.com/Byh0ki) in https://github.com/TwiN/gatus/pull/1175
- chore(deps): bump github.com/valyala/fasthttp from 1.62.0 to 1.64.0 by [@&#8203;dependabot](https://github.com/dependabot)\[bot] in https://github.com/TwiN/gatus/pull/1162
- chore(deps): bump github.com/miekg/dns from 1.1.66 to 1.1.67 by [@&#8203;dependabot](https://github.com/dependabot)\[bot] in https://github.com/TwiN/gatus/pull/1166

#### New Contributors

- [@&#8203;medaved](https://github.com/medaved) made their first contribution in https://github.com/TwiN/gatus/pull/928
- [@&#8203;perfectra1n](https://github.com/perfectra1n) made their first contribution in https://github.com/TwiN/gatus/pull/1163
- [@&#8203;jasonshugart](https://github.com/jasonshugart) made their first contribution in https://github.com/TwiN/gatus/pull/1134
- [@&#8203;joy4eg](https://github.com/joy4eg) made their first contribution in https://github.com/TwiN/gatus/pull/1165
- [@&#8203;Byh0ki](https://github.com/Byh0ki) made their first contribution in https://github.com/TwiN/gatus/pull/1175
- [@&#8203;olexandrd](https://github.com/olexandrd) made their first contribution in https://github.com/TwiN/gatus/pull/1180

**Full Changelog**: https://github.com/TwiN/gatus/compare/v5.20.0...v5.21.0

</details>

---

### Configuration

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♻ **Rebasing**: Whenever PR is behind base branch, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

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---

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Reviewed-on: #1105
Co-authored-by: Renovate Bot <renovate-bot@alexlebens.net>
Co-committed-by: Renovate Bot <renovate-bot@alexlebens.net>

clusters/cl01tl/monitoring/gatus/values.yaml

@@ -7,7 +7,7 @@ gatus:
     enabled: true
   image:
     repository: ghcr.io/twin/gatus
-    tag: v5.20.0
+    tag: v5.21.0
   annotations:
     reloader.stakater.com/auto: "true"
   service: