Problem/Motivation
We unified the uid base field definitions and deprecated various default value callbacks in #2949964: Add an EntityOwnerTrait to standardize the base field needed by EntityOwnerInterface.
On a site today, we noticed a bug with some media types because the default value callback in their uid base field override still referenced media_entity. That made me think and investigate and I also found several sites where the default value callback still uses Drupal\node\Entity\Node::getCurrentUserId, Drupal\media_entity\Entity\Media::getCurrentUserId and so on, which would fail on Drupal 9 (no default value which causes an SQL exception).
Proposed resolution
Update all possibly existing base field overrides for known entity types, fix their default value callbacks.