This is how I fixed it.
I went to about.gitlab.com/releases/categories/releases and looked up the latest 12.10.x release, which was 12.10.8.
Then I typed
apt install gitlab-ce=12.10.8-ce.0
if you happen to have the enterprise edition installed, you might want to change the ce to ee:
apt install gitlab-ee=12.10.8-ee.0
After that I could update to the latest gitlab version by simply typing
apt install gitlab-ce
or, again, if you have the enterprise edition installed, change the ce to ee:
apt install gitlab-ee