Updated testing from Ubuntu 18+22 to 20+22.

[MichaelClerx]

• Ubuntu 18 is no longer supported https://ubuntu.com/18-04
• We specify testing on 18.04 and "latest", which used to mean 18+20, but now means 18+22
• Tests no longer run on 18
• This PR drops 18.04 testing, and adds 20.04 testing

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (5c4bd01) 100.00% compared to head (7c99d81) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1485 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 97 97 Lines 9530 9530 ========================================= Hits 9530 9530 ```

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