Replace ember-lts-6.4 and ember-lts-6.8 scenarios with ember-lts-6.12 (#559)

6.12 is the current LTS that matches the ember-source version used
by the test-app. 6.4 and 6.8 are older LTS versions that are no
longer necessary to test against.

Co-authored-by: NullVoxPopuli <199018+NullVoxPopuli@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

.github/workflows/ci.yml

@@ -62,8 +62,7 @@ jobs:
           - ember-lts-3.28
           - ember-lts-4.12
           - ember-lts-5.12
-          - ember-lts-6.4
-          - ember-lts-6.8
+          - ember-lts-6.12
           - ember-release
           - ember-beta
           - ember-canary

test-app/config/ember-try.js

@@ -40,18 +40,10 @@ module.exports = async function () {
         },
       },
       {
-        name: 'ember-lts-6.4',
+        name: 'ember-lts-6.12',
         npm: {
           devDependencies: {
-            'ember-source': '~6.4.0',
-          },
-        },
-      },
-      {
-        name: 'ember-lts-6.8',
-        npm: {
-          devDependencies: {
-            'ember-source': '~6.8.0',
+            'ember-source': '~6.12.0',
           },
         },
       },