set artifact lifetime explicitly (default is buggy)

Some time back we decided to limit the lifetime of CI artifacts to prevent the project from getting too big as we already had one gitlab outage before Christmas due to missing server space. The default setting in .gitlab-ci.yml seems to be buggy as still artifacts are stored for the default time (4 weeks) everywhere. Now looking at https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/usage_quotas, we have 14GB of artifact space in use, so I just set some explicit values to what I hope are sensible defaults.

Fyi ci people: @sagnowski @norvell @pihlajakuja @pajunen @zhaokke