ENH: Deprecate `.ini` configurations.

[tobiasraabe]

Is your feature request related to a problem?

#253 added support for pyproject.toml. With v0.3 or v1 we will deprecate all ini formats.

Describe the solution you'd like

• Remove parsing related to .ini codes.
• Use the approach by ward to read in the configuration and overwrite cli defaults.
• Remove defaults from help strings and add them automatically in _pytask.click.
• Warnings for unknown config values.

API breaking implications

.ini formats will be deprecated.