fix(deps): update dependency @hello.nrfcloud.com/proto to v15.3.21 (#327)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package-lock.json

@@ -57,7 +57,7 @@
     "yaml": "2.6.0"
   },
   "dependencies": {
-    "@hello.nrfcloud.com/proto": "15.3.19",
+    "@hello.nrfcloud.com/proto": "15.3.21",
     "@hello.nrfcloud.com/proto-map": "16.1.19",
     "@sinclair/typebox": "0.33.22",
     "e118-iin-list": "4.1.4",