Bump immutable from 5.0.3 to 5.1.5 in /web/vtadmin

Bumps [immutable](https://github.com/immutable-js/immutable-js) from 5.0.3 to 5.1.5.
- [Release notes](https://github.com/immutable-js/immutable-js/releases)
- [Changelog](https://github.com/immutable-js/immutable-js/blob/main/CHANGELOG.md)
- [Commits](immutable-js/immutable-js@v5.0.3...v5.1.5)

---
updated-dependencies:
- dependency-name: immutable
  dependency-version: 5.1.5
  dependency-type: indirect
...

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

Author: dependabot[bot]