build: pin packageManager to pnpm@11.0.6

action-setup infers the pnpm version from this field. The test job
doesn't check out an upstream repo, so without it action-setup errors
'No pnpm version is specified'.

Author: JohnMcLear

package.json

@@ -28,6 +28,7 @@
     "type": "individual",
     "url": "https://etherpad.org/"
   },
+  "packageManager": "pnpm@11.0.6",
   "scripts": {
     "lint": "eslint .",
     "lint:fix": "eslint --fix .",