Was the key fetched per the install instructions?

curl -fsSL https://www.mongodb.org/static/pgp/server-4.4.asc | \
   sudo gpg -o /usr/share/keyrings/mongodb-server-4.4.gpg \
   --dearmor

This expires May 2027.

I just did a clean install using the 4.4 repo on Focal. Also a 4.2 to 4.4 on Bionic.

The recommended way is to upgrade in place, some have success using dump/restore though.