gitlab-ci: switch from Ubuntu 16.04 to 20.04

Ubuntu 16.04 is past its normal LTS lifespan, so let's switch to Ubuntu
20.04 instead, which is the latest regular LTS version.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>

Author: bk2204

.gitlab-ci.yml

@@ -35,7 +35,7 @@ test:linux:
   parallel:
     matrix:
       - jobname: linux-old
-        image: ubuntu:16.04
+        image: ubuntu:20.04
         CC: gcc
       - jobname: linux-sha256
         image: ubuntu:latest