Add check for git in remediation

What does this MR do?

Add git_available? to Remediations::Collection.

This was raising an error in UI when the operating system is supported but git is not. Empty objects in the remediation array were causing the parsing error.

Screenshot

Before	After

What are the relevant issue numbers?

gitlab-org/gitlab#408792 (closed)

Does this MR meet the acceptance criteria?

• Changelog trailer added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer