Add a warning for Keycloak URL changes after 8.5

[conceptualshark]

Description

In 8.5, a change to Identity to use the frontend URL instead of the backend url was not documented. In some cases, users may be blocking the frontend URL, causing impacts to functionality.

This PR adds the warning to the 8.5 announcements and the 8.4 - > 8.5 upgrade guides, and adds the note for visibility in the Keycloak connection guides for 8.5 and 8.6.

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.

[github-actions]

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

These files were changed only in versioned_docs/version-8.5/. You might want to duplicate these changes in docs/.

• versioned_docs/version-8.5/reference/announcements.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.

[conceptualshark]

Assuming there's no config change needed, I think this represents the best places to add the note. Since it was missed in the initial announcements, I've added it to both the 8.5 and /next connection guide, so it isn't lost in prior update guides users may not think apply to them.

[mesellings]

Lgtm - just a suggestion about addressing the customer directly, but feel free to ignore 👍

[mesellings]

Lgtm 👍 🚀