Fix: Add warning about removing custom OAuth2 grant types [4.1.0]

[wso2-engineering-bot]

This PR was automatically generated by Claude AI.

• Issue: A warning should be added about removing custom OAuth2 grant types #10217
• Type: Documentation
• Summary: Add warning about removing custom OAuth2 grant types from deployment.toml and the importance of cleaning up affected applications first
• Style Scope Verification: Microsoft Style Guidelines have been applied ONLY to newly added content without modifying existing content style unless specifically requested.
• Verification: The added content follows proper Markdown and MkDocs admonitions syntax. Build verification could not be completed due to Python version compatibility with older mkdocs version, but the syntax is validated as correct.