Change GHA runners to ubuntu-latest

This is a hail-mary change: we are experiencing problems with GHA
runs timing out (behave tests taking up to five hours to run). We
were running on a not-current GHA runner version, and I think the
old runners get fewer resources than the newer ones.

Signed-off-by: Jonathan Dowland <[email protected]>

Author: jmtd

.github/workflows/gendocs.yml

@@ -20,7 +20,7 @@ concurrency:
 jobs:
   gendocs:
     name: Generate documentation
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
     steps:

.github/workflows/image-workflow-template.yml

@@ -10,7 +10,7 @@ env:
 jobs:
   openjdkci:
     name: OpenJDK S2I Build and Test
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
     steps: