Update go to 1.21; Close #6

[dark-person]

Changes Made

• Update go.mod to go 1.21
• Change Error Message in GUI to display Uppercase in 1st Letter
• Vaildate selected folder structure (close Validate Image Directory Structure before export/confirm #6 )
• Add back the example.xml for package parser testing
• Change scanner package function include error in returned value

Reason of Changes

• Using updated golang for better development
• Make Error Message more readable
• Implement checking to improve UX
• For package parser testing
• To Provide Error Message to User

Checklist:

• I have run the new code and ensure the change is work expected
• I have write/modify comments to important function & hard-to-understand code
• I have create/modify corresponding test for new changes
• I have made corresponding changes to the documentation
• I have run all new & existing test and pass locally with my changes
• I have checked my code has no misspellings & no warnings
• I have checked that only essential code remains in this pull request