Update CI to latest Ubuntu image. (#3428)

Author: BenZickel

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ env:
 
 jobs:
   lint:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: [3.8]
@@ -35,7 +35,7 @@ jobs:
       run: |
         make lint
   docs:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: lint
     strategy:
       matrix:
@@ -63,7 +63,7 @@ jobs:
           make docs
           make doctest
   tutorials-html:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: lint
     strategy:
       matrix:
@@ -97,7 +97,7 @@ jobs:
         run: |
           SPHINXOPTS="-E" make tutorial
   unit:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: docs
     strategy:
       matrix:
@@ -129,7 +129,7 @@ jobs:
           COVERALLS_PARALLEL: true
           COVERALLS_FLAG_NAME: ${{ matrix.test-name }}
   examples:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: [docs, tutorials-html]
     strategy:
       matrix:
@@ -165,7 +165,7 @@ jobs:
           COVERALLS_PARALLEL: true
           COVERALLS_FLAG_NAME: ${{ matrix.test-name }}
   integration_1:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: docs
     strategy:
       matrix:
@@ -197,7 +197,7 @@ jobs:
           COVERALLS_PARALLEL: true
           COVERALLS_FLAG_NAME: ${{ matrix.test-name }}
   integration_2:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: docs
     strategy:
       matrix:
@@ -229,7 +229,7 @@ jobs:
           COVERALLS_PARALLEL: true
           COVERALLS_FLAG_NAME: ${{ matrix.test-name }}
   funsor:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: docs
     strategy:
       matrix:
@@ -264,7 +264,7 @@ jobs:
           COVERALLS_FLAG_NAME: ${{ matrix.test-name }}
   finish:
     needs: [unit, examples, integration_1, integration_2, funsor]
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: [3.8]