feat(broker): update docs in backups/GCS regarding initialization and added update guide from 8.6 to 8.7

[entangled90]

Description

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

closes #4410

[github-actions]

👋 🤖 🤔 Hello! Did you make your changes in all the right places?

These files were changed only in docs/. You might want to duplicate these changes in versioned_docs/version-8.6/.

• docs/self-managed/operational-guides/backup-restore/zeebe-backup-and-restore.md
• docs/self-managed/operational-guides/update-guide/860-to-870.md

You may have done this intentionally, but we wanted to point it out in case you didn't. You can read more about the versioning within our docs in our documentation guidelines.

[deepthidevaki]

Suggested change

	To prevent misconfiguration, Zeebe will check at startup that the specified bucket exists and can be accessed, but
	it will not prevent Zeebe to start up.
	To prevent misconfiguration, Zeebe will check at startup that the specified bucket exists and can be accessed, and log at WARN level if the bucket does not exist.

[deepthidevaki]

🚀

[akeller]

Looks like this merged with a typo @entangled90. Please be sure to include tech-writers in your reviewers for most docs changes.