Introduces finalize stage for security release

What does this MR do and why?

Adds the required CI configuration to include the jobs associated with the final steps of a security release. At the mooment, this only includes a job notifying the final steps in the security pipeline have been triggered.

The rest of the security CI files were also adjusted so they can run as a single pipeline.

Additionally, this commit refactors the Security::Prepare::Start service into a re-usable slack class

Related to gitlab-com/gl-infra/delivery#19434 (closed)

Tests

Pipelines

With SECURITY_RELEASE_PIPELINE='prepare'	With SECURITY_RELEASE_PIPELINE='finalize'	With SECURITY_RELEASE_PIPELINE='true'
Pipeline	Pipeline	Pipeline

With SECURITY_RELEASE_PIPELINE='true' running state

Slack output

Start stage	Finalize

Author Check-list

• [-] Has documentation been updated?