chore: change webhook url for releases

[hugohaggmark]

What is this feature?

Updates the Slack webhook URL secret reference in the release workflow.

Why do we need this feature?

So release notifications get posted to the correct Slack channel.

Who is this feature for?

Grafana maintainers

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[github-actions]

Hello! 👋 This repository uses Auto for releasing packages using PR labels.

✨ This PR can be merged. It will not be considered when calculating future versions of the npm packages and will not appear in the changelogs.

[Copilot]

Pull request overview

Updates the Slack webhook secret reference used by the release job in the main CI workflow so that release notifications are sent to the intended Slack channel.

Changes:

• Switches the Vault secret key used to populate SLACK_WEBHOOK_URL for the release workflow (slack_webhook_url_fp instead of slack_webhook_url).

[jackw]

LGTM! 🚀