Update compatible Redmine version to 6.0

[gemmaro]

Hello,

This update makes the plugin compatible with Redmine version 6.0.

To achieve this,

• it conditionally calls the unloadable method,
• updates test cases to accommodate newer Rails API changes,
• updates the Redmine version requirement, and
• enables setting the PlantUML executable path during testing.

Thank you,
gemmaro.