Adds `managed` boolean column to Clusters table

What does this MR do?

Adds managed boolean column to Clusters table. This new column will be used to distinguish those clusters that are automatically managed by GitLab.

Note: managed? term was renamed on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26567 to provided_by_user?

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/56557

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer

Closes #56557 (closed)