build(deps): bump slackapi/slack-github-action from 2.1.0 to 3.0.1

Bumps [slackapi/slack-github-action](https://github.com/slackapi/slack-github-action) from 2.1.0 to 3.0.1.
- [Release notes](https://github.com/slackapi/slack-github-action/releases)
- [Commits](slackapi/slack-github-action@b0fa283...af78098)

---
updated-dependencies:
- dependency-name: slackapi/slack-github-action
  dependency-version: 3.0.1
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/gardener-investigate-issue.yml

@@ -49,7 +49,7 @@ jobs:
       - name: Post investigation start to Slack
         id: start_slack
         continue-on-error: true
-        uses: slackapi/slack-github-action@b0fa283ad8fea605de13dc3f449259339835fc52 # v2.1.0
+        uses: slackapi/slack-github-action@af78098f536edbc4de71162a307590698245be95 # v3.0.1
         with:
           method: chat.postMessage
           token: ${{ secrets.SLACK_GARDENER_BOT_TOKEN }}
@@ -218,7 +218,7 @@ jobs:
 
       - name: Post to Slack
         if: always() && steps.slack_payload.outputs.result
-        uses: slackapi/slack-github-action@b0fa283ad8fea605de13dc3f449259339835fc52 # v2.1.0
+        uses: slackapi/slack-github-action@af78098f536edbc4de71162a307590698245be95 # v3.0.1
         with:
           method: chat.postMessage
           token: ${{ secrets.SLACK_GARDENER_BOT_TOKEN }}