v2021: Default ospack recipes to stable

Now that v2021 is stable it makes sense to make the recipe do the most appropriate thing by default to get consistent results.

This also fixes an issue with the test executing the sample recipes which was failing with:

apt | The following packages have unmet dependencies:
apt |  evolution-data-server : Depends: libcanberra-gtk3-0 (>= 0.25) but it is not going to be installed
apt |                          Depends: libcanberra0 (>= 0.2) but it is not going to be installed
apt |  pulseaudio : Depends: libtdb1 (>= 1.2.7+git20101214) but it is not installable
apt | E: Unable to correct problems, you have held broken packages.

This was caused by the removal of the LGPL-3 libtdb package from the main repository, while the updated libcanberra and pulseaudio with the dependency dropped have only been pushed to the v2021-updates repository which is only enabled for stable releases.

Task: T7879