[SplashScreen] Mark sync methods as Obsolete

[MarvinKlein1508]

Pull Request

📖 Description

This PR marks all synchronous ShowSplashScreen methods as obsolete because they won't be in v5 anymore. In addition it also removes the broken sync SplashScreen samples from the demo site. I also updated the docs section with a warning for the synchrounous methods.

🎫 Issues

Related to #4637

👩‍💻 Reviewer Notes

This change will help users to switch to the async functions early on which will make the migration to v5 easier later on.

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

⏭ Next Steps