Update CI to use Ubuntu 20.04

**Is your feature request related to a problem? Please describe.**
Ubuntu 18.04 EOL is approaching
cFS was already updated in nasa/cfs#559, cFE should match

**Describe the solution you'd like**
Update to use Ubuntu 20.04 in CI

**Describe alternatives you've considered**
None

**Additional context**
None

**Requester Info**
Jacob Hageman - NASA/GSFC