Let's use a real _service file.

NOTE: the version is a small downgrade, but that's because I use %cd (aka commit date) as version identifier.
in the _service file I used the same commit date, so in fact this is the same source.

- Convert to use a proper git source _service:
+ To update, one just needs to update the commit/revision in the
_service file and run `osc service dr`.
+ The version of the package is defined by the commit date of the
revision, followed by the abbreviated git hash (The same
revision used before results thus in a downgrade to 20210118,
but as this is a alltime new package, this is acceptable.