CI: upgrade from ubuntu-18.04 (#1638)

Author: iliana

.github/workflows/docker-image.yml

@@ -6,7 +6,7 @@ name: docker-image
 on: push
 jobs:
   docker-image:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
       # actions/checkout@v2
       - uses: actions/checkout@28c7f3d2b5162b5ddd3dfd9a45aa55eaf396478b

.github/workflows/rust.yml

@@ -7,7 +7,7 @@ on: [ push, pull_request ]
 
 jobs:
   check-style:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     # actions/checkout@v2
     - uses: actions/checkout@28c7f3d2b5162b5ddd3dfd9a45aa55eaf396478b
@@ -23,7 +23,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-18.04, macos-11 ]
+        os: [ ubuntu-20.04, macos-11 ]
     steps:
     # actions/checkout@v2
     - uses: actions/checkout@28c7f3d2b5162b5ddd3dfd9a45aa55eaf396478b
@@ -39,7 +39,7 @@ jobs:
       run: cargo run --bin omicron-package -- check
 
   clippy-lint:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     # actions/checkout@v2
     - uses: actions/checkout@28c7f3d2b5162b5ddd3dfd9a45aa55eaf396478b
@@ -64,7 +64,7 @@ jobs:
   # This is just a test build of docs.  Publicly available docs are built via
   # the separate "rustdocs" repo.
   build-docs:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     # actions/checkout@v2
     - uses: actions/checkout@28c7f3d2b5162b5ddd3dfd9a45aa55eaf396478b