Remove testing ember release/beta/canary (now in v6)

Author: NullVoxPopuli

.github/workflows/ci.yml

@@ -60,9 +60,6 @@ jobs:
           - ember-3.25
           - ember-3.28-lts
           - first-inert
-          - ember-release
-          - ember-beta
-          - ember-canary
           - ember-could-get-used-to-this
           - embroider-safe
           - embroider-optimized