Add a warning about including `mintAdminToken`

Thanks for these amazing snippets but I feel like there should be a warning about including this in any production environment since the combination of `mintAdminToken` and `recursiveDelete` would enable to anyone blasting an entire production database.

I suggest you create an example that isn't copy-pasteable.

https://github.com/firebase/snippets-node/blob/ea3747eee572d3e1bebd1cdafda9fa34a6dc38f0/firestore/solution-deletes/functions/index.js#L19

Start with firestore/solution-deletes/functions/index.js at the linked example around line 19, and inspect how mintAdminToken and recursiveDelete are presented. Make the production risk explicit and ensure the example is not directly copy-pasteable; confirm the resulting snippet still communicates the deletion workflow safely.

Written by the indexing model from the issue text.