Backup-DbaDbMasterKey - Output filename property with file extention

[andreasjordan]

Type of Change

• [x] Bug fix (non-breaking change, fixes #9339 )
• [ ] New feature (non-breaking change, adds functionality, fixes # )
• [ ] Breaking change (affects multiple commands or functionality, fixes # )
• [ ] Ran manual Pester test and has passed (.\tests\manual.pester.ps1)
• [ ] Adding code coverage to existing functionality
• [ ] Pester test is included
• [ ] If new file reference added for test, has is been added to github.com/dataplat/appveyor-lab ?
• [ ] Unit test is included
• [ ] Documentation
• [ ] Build system

I aligned the code with the code of Backup-DbaServiceMasterKey.

See issue for details.

[potatoqualitee]

lookin good 👀