Update SECURITY.md with current security policy and a new way to report vulnerabilities

[Xerkus]

Our currently adopted security policy is to provide security fixes or backports to all previous releases as long as their minimum PHP version is in active or security maintenance.

This is not reflected in SECURITY.md in .github repositories or on website at https://getlaminas.org/security/

Recently GitHub provided beta feature to privately report security vulnerabilities using GitHub itself. This is now enabled on all our repositories. Consequently SECURITY.md needs to be updated with instructions on how to use it. See https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability