Adjust last step on security developer template

What does this MR do?

Adjust last step on security developer template to remove the and **when all backports including the MR targeting master are ready.**

That part was introduced by !28339 (merged) requiring all merge requests to be assigned at once, this was added momentarily to prevent accidental merges. Now that we have automatic merge request validation this part shouldn't be necessary, it's also causing confusion on developers https://gitlab.com/gl-retrospectives/create-stage/ecosystem/-/issues/23#note_619064955

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.