Widen test matrix to include ember-source 5.8

Co-Authored-By: Brad Overton <[email protected]>

Author: NullVoxPopuli

.github/workflows/ci.yml

@@ -74,6 +74,7 @@ jobs:
           - ember-lts-4.8
           - ember-lts-4.12
           - ember-lts-5.4
+          - ember-lts-5.8
           - ember-concurrency-4.0
           - ember-release
           - ember-beta

tests/test-app/config/ember-try.js

@@ -41,6 +41,14 @@ module.exports = async function () {
           },
         },
       },
+      {
+        name: 'ember-lts-5.8',
+        npm: {
+          devDependencies: {
+            'ember-source': '~5.8.0',
+          },
+        },
+      },
       {
         name: 'ember-release',
         npm: {