create-app: remove root diff command

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/dull-spoons-doubt.md

@@ -0,0 +1,11 @@
+---
+'@backstage/create-app': patch
+---
+
+Removed usage of the deprecated `diff` command in the root `package.json`.
+
+To make this change in an existing app, make the following change in the root `package.json`:
+
+```diff
+-    "diff": "lerna run diff --",
+```

packages/create-app/templates/default-app/package.json.hbs

@@ -14,7 +14,6 @@
     "tsc": "tsc",
     "tsc:full": "tsc --skipLibCheck false --incremental false",
     "clean": "backstage-cli repo clean",
-    "diff": "lerna run diff --",
     "test": "backstage-cli test",
     "test:all": "lerna run test -- --coverage",
     "lint": "backstage-cli repo lint --since origin/master",