Release feature: Link Jira issues to GitLab deployments

What does this MR do and why?

This MR removes the jira_deployment_issue_keys feature flag to release the feature of linking Jira issues to GitLab deployments.

• #300031 (closed)
• #415025 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #300031 (closed)