Switch UbuCon demo to Java modernization

[justinyoo]

Summary

Update UbuCon Demo 1 to present the Java modernization workflow instead of the .NET variant. This activates the Java-specific slide titles, sandbox command, assessment copy, and modernization prompts while retaining the .NET content as commented alternatives.

Change type

• Presentation content
• Presentation design
• PowerShell
• Bash
• Bicep
• Documentation or repository maintenance

Validation

• Marp HTML generation succeeds.
• Updated slides were visually reviewed.
• PowerShell files parse without errors.
• Bash files pass bash -n.
• Bicep files build successfully.
• No generated files, credentials, or sensitive data are included.

Additional checks:

• Verified the rendered UbuCon deck shows the Java Demo 1 title.
• Verified the java-appmod sandbox command is rendered.
• Verified the Java 21 and Spring Boot 4.1 prompt is rendered.
• Verified the .NET Demo 1 heading is not visible.

Screenshots

Not included.

Related issues

None.