from the man page, under Debian, it seems to be ca-certificates.crt.
Note that simply moving the broken symlinks out of the way fixed the issue for Julia. I’m not sure what the mbedTLS library that seems to be used by Julia needs.
After discovering the origin of those symlinks, I ran the update-ca-certificates script as root. The ca-certificates.crt timestamp seems to have been updated today a little while ago, so at some point it has been recreated, either when I ran the script, or automagically by the mbedTLS lib, not sure exactly which one of my actions was the direct cause of that.