Add a validation to prevent closing security implementation issues

On #1106 (closed), we changed the way security implementation issues are closed: MR's targeting master are merged earlier and backports are merged during the Security Release, as a result closing security implementation issues is now one of the last steps on the Security Release.

We should add a validation to prevent closing security implementation issues once the MR targeting master is merged, e.g. we need to prevent Closes #123 on the MR description (the one targeting master), being 123 the security implementation issue IID