Update dependency @slack/webhook to v7.0.9 (TryGhost#27452)

This PR contains the following updates:

| Package | Change |
[Age](https://docs.renovatebot.com/merge-confidence/) |
[Confidence](https://docs.renovatebot.com/merge-confidence/) |
|---|---|---|---|
| [@slack/webhook](https://docs.slack.dev/tools/node-slack-sdk/webhook/)
([source](https://redirect.github.com/slackapi/node-slack-sdk)) |
[`7.0.8` →
`7.0.9`](https://renovatebot.com/diffs/npm/@slack%2fwebhook/7.0.8/7.0.9)
|
![age](https://developer.mend.io/api/mc/badges/age/npm/@slack%2fwebhook/7.0.9?slim=true)
|
![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/@slack%2fwebhook/7.0.8/7.0.9?slim=true)
|

---

### Release Notes

<details>
<summary>slackapi/node-slack-sdk (@&TryGhost#8203;slack/webhook)</summary>

###
[`v7.0.9`](https://redirect.github.com/slackapi/node-slack-sdk/releases/tag/%40slack/webhook%407.0.9)

[Compare
Source](https://redirect.github.com/slackapi/node-slack-sdk/compare/@slack/webhook@7.0.8...@slack/webhook@7.0.9)

##### Patch Changes

-
[`3a9c444`](https://redirect.github.com/slackapi/node-slack-sdk/commit/3a9c444):
build(deps): bump minimum axios version to 1.15.0

</details>

---

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ghost/core/package.json

@@ -73,7 +73,7 @@
     "@faker-js/faker": "7.6.0",
     "@isaacs/ttlcache": "1.4.1",
     "@sentry/node": "7.120.4",
-    "@slack/webhook": "7.0.8",
+    "@slack/webhook": "7.0.9",
     "@tryghost/adapter-base-cache": "0.1.23",
     "@tryghost/admin-api-schema": "4.7.2",
     "@tryghost/api-framework": "1.0.7",