Spring Zeebe SDK - Getting Started - Removal of unnecessary identity repo setup

[megglos]

Description

This closes #4495 with the following changes:

• removal of the identity repo setup for 8.5+ docs, it's not needed as the Spring Zeebe SDK has no dependency on identity artifacts
• inline of the maven setup for the sdk in the Getting started with Spring guide - alternative is to just link to the separate page but I figured this would just interrupt the user reading the guide and I went for inlining the dependency to add

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.