build(deps): bump sortablejs from 1.15.3 to 1.15.6 in /webapp

Bumps [sortablejs](https://github.com/SortableJS/Sortable) from 1.15.3 to 1.15.6.
- [Release notes](https://github.com/SortableJS/Sortable/releases)
- [Commits](https://github.com/SortableJS/Sortable/compare/1.15.3...1.15.6)

---
updated-dependencies:
- dependency-name: sortablejs
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

webapp/package.json

@@ -17,7 +17,7 @@
     "bootstrap": "^5.3.3",
     "bootstrap-icons-vue": "^1.11.3",
     "mitt": "^3.0.1",
-    "sortablejs": "^1.15.3",
+    "sortablejs": "^1.15.6",
     "spark-md5": "^3.0.2",
     "vue": "^3.5.12",
     "vue-i18n": "10.0.4",