Travis: Change to run on Ubuntu focal

Author: df7cb

.travis.yml

@@ -11,7 +11,7 @@ env:
   - PG_SUPPORTED_VERSIONS=16
 
 language: C
-dist: bionic
+dist: focal
 
 before_install:
   # extra apt.pg.o.sh options added in version 204, travis currently has 199 (2019-11-27)