CVE-2026-22822: External Secrets Operator insecurely retrieves secrets through the getSecretKey templating function

The getSecretKey template function, while introduced for senhasegura Devops Secrets Management (DSM) provider, has the ability to fetch secrets cross-namespaces with the roleBinding of the external-secrets controller, bypassing our security mechanisms.

This function was completely removed, as everything done with that templating function can be done in a different way while respecting our safeguards (for example, using sourceRef like explained here: https://github.com/external-secrets/external-secrets/issues/5690#issuecomment-3630977865)