Test 22a, update CircleCI images, rename default branch (#5)

* Updated testing
* Update CircleCI configuration to test on 22a
* Update CircleCI configuration to migrate from deprecated base images to docker convenience containers

.circleci/config.yml

@@ -5,7 +5,7 @@ orbs:
 jobs:
   build-latest:
     machine:
-      image: ubuntu-1604:201903-01
+      image: ubuntu-2004:current  
     steps:
         - checkout
         - matlab/install:
@@ -15,9 +15,21 @@ jobs:
         - store_test_results:
             path: test-results
 
+  build-21b:
+    machine:
+      image: ubuntu-2004:current
+    steps:
+        - checkout
+        - matlab/install:
+            release: R2021b
+        - matlab/run-tests:
+            test-results-junit: test-results/matlab/results.xml
+        - store_test_results:
+            path: test-results
+
   build-21a:
     machine:
-      image: ubuntu-1604:201903-01
+      image: ubuntu-2004:current
     steps:
         - checkout
         - matlab/install:
@@ -29,7 +41,7 @@ jobs:
 
   build-20b:
     machine:
-      image: ubuntu-1604:201903-01
+      image: ubuntu-2004:current
     steps:
         - checkout
         - matlab/install:
@@ -41,7 +53,7 @@ jobs:
 
   build-20a:
     machine:
-      image: ubuntu-1604:201903-01
+      image: ubuntu-2004:current
     steps:
         - checkout
         - matlab/install:
@@ -56,6 +68,7 @@ workflows:
   multiple-matlabs:
     jobs:
       - build-latest
+      - build-21b
       - build-21a
       - build-20b
       - build-20a