fix(deps): bump ep_plugin_helpers range to ^0.6.1 (the published version) (#143)

Follow-up to the toolbar-select migration (#142). Etherpad's plugin
installer resolves the dep range's lower-bound version literally —
`^0.6.0` makes it ask npm for `ep_plugin_helpers@0.6.0`, which was
never published (the auto-publish workflow bumped straight to `0.6.1`,
skipping 0.6.0). CI on main has been red since the merge for this
reason. Anchor to the actual published version so the install resolves.

Author: JohnMcLear

package.json

@@ -28,7 +28,7 @@
     "url": "https://etherpad.org/"
   },
   "dependencies": {
-    "ep_plugin_helpers": "^0.6.0"
+    "ep_plugin_helpers": "^0.6.1"
   },
   "devDependencies": {
     "eslint": "^8.57.1",