make `load_stac` official

[clausmichele]

The load_stac process is still in the proposal folder, it should be moved up with the other official processes in my opinion. It's supported by the CDSE, VITO, EODC, Eurac and probably other back-ends.

I did try to move it already in an old PR @m-mohr , is there a reason for keeping it under proposals?

[m-mohr]

See #544, the main reason is that there are still some open issues related to load_stac in the issue tracker.

All processes in here are "official", they are just categorized into "stable" and "experimental" (means changes are likely, breaking changes may occur). In proposal we can do breaking changes without going to v3, once stable we need to issue v3 (which is unlikely anytime soon). So unless we are certain to not make breaking changes, we should probably keep it in proposals. Looking at the open issues, they don't look breaking on first sight, but haven't looked into details. If we don't think anything will break while resolving the other issues, we can consider moving it to stable.