chore: remove references to ubuntu 18.04 ahead of EOL

While we will still build for 18.04, we will now stop testing against it. Instead, we will test against 20.04, the next EOL release after 18.04.

Once Ubuntu 18.04 is EOL, any references to it will also be removed.

Note that 18.04 support was already deprecated in Dokku 0.28.0, so this change is just acknowledging that deprecation.

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ on:
 jobs:
   build:
     name: build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     strategy:
@@ -49,7 +49,7 @@ jobs:
   unit-tests:
     name: unit.${{ matrix.index }}
     needs: build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: ${{fromJson(needs.build.outputs.matrix)}}
@@ -89,7 +89,7 @@ jobs:
   docker-deploy-tests:
     name: docker
     needs: build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
 
@@ -115,7 +115,7 @@ jobs:
   go-tests:
     name: go.${{ matrix.index }}
     needs: build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
@@ -156,7 +156,7 @@ jobs:
   publish-test-results:
     name: publish-test-results
     needs: unit-tests
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     # the build-and-test job might be skipped, we don't need to run this job then
     if: success() || failure()