Lower minimum AU version back to 2.15.0

The test app uses 2.17.0 as this allows us to use the new scss import
paths. 2.15.0 is supported if the client app imports the scss from the
old paths. That makes this set of changes non-breaking.

.changeset/tall-tools-wink.md

@@ -3,5 +3,4 @@
 ---
 
 Update icon usage to inline SVGs instead of using svgiconset.
-BREAKING: update minimum supported ember-appuniversum version to 2.17.0
 If using ember-appuniversum version greater than 3.4.1, icon components are now used instead of string icon names, this uses inline SVGs that can be supported in unusual hosting environments.

package.json

@@ -211,7 +211,7 @@
     "webpack": "^5.78.0"
   },
   "peerDependencies": {
-    "@appuniversum/ember-appuniversum": "^2.17.0 || ^3.0.0",
+    "@appuniversum/ember-appuniversum": "^2.15.0 || ^3.0.0",
     "@glint/template": "^1.1.0",
     "ember-changeset": "^4.1.2",
     "ember-cli-sass": "^11.0.1",