Change from kingpin in the client package breaks automation

Impact on the user
Automation is broken and cannot be used with >=2.31.0

Steps to reproduce

• Use 2.30.0
• Install the client
• Create a file to contain all flags
• Add a service, e.g. pmm-admin add mysql @/path/to/file
• Remove service
• Upgrade client to 2.31.0
• Add a service, e.g. pmm-admin add mysql @/path/to/file

Actual result
Failure to register the service

Expected Result
Successfully registering the service

Workaround
N/A

Details
A flags file is used for the following reasons:

• it allows detection of a change in configuration via automation
• it hides secrets from the command line
• it works with all technologies allowing a single template to render the file for use
• it avoids convoluted ways to support automating all kinds of configurations