Remove requirement to target security branches
As part of https://gitlab.com/gitlab-org/release/framework/issues/165 the Delivery team discussed the removal of the security branches for security releases. This simplifies the work done by release managers, and makes it easier to automate more parts of the security release workflow.
The proposal to remove security branches was approved in https://gitlab.com/gitlab-org/release/framework/issues/165#note_138139016.