docs(security): promote GitHub private reporting, fix contact email

The security-opensource@gruposantander.com mailbox was undeliverable; use opensource@gruposantander.com and make GitHub Private Vulnerability Reporting (now enabled) the preferred channel.

Author: opensource-SantanderAI

README.md

@@ -187,7 +187,7 @@ and [Code of Conduct](CODE_OF_CONDUCT.md) before getting started.
 
 Please report security vulnerabilities responsibly. See our [Security Policy](SECURITY.md)
 for how to report (do **not** open a public issue for vulnerabilities). Contact:
-**security-opensource@gruposantander.com** or use GitHub Security Advisories.
+**opensource@gruposantander.com** or use GitHub Security Advisories.
 
 ## License