Delete obsolete Snyk workflow .github/workflows/snyk.yml

As of January 2025, with guardian/.github#96, this workflow file is completely obsolete, and has no effect on GitHub repositories - other than to make the main code branch [appear broken](guardian/service-catalogue#1749)! Consequently, this PR deletes this file.

Please feel free to approve & merge this PR - or, if appropriate, [archive](https://docs.github.com/en/repositories/archiving-a-github-repository/archiving-repositories) the whole repository.

Failing that, this PR will be **automatically merged** on Wednesday 10th December 2025.

Contact @rtyley with any questions/comments!

Author: delete-old-snyk-files[bot]