Renames Cluster#managed? to provided_by_user?

What does this MR do?

First step of https://gitlab.com/gitlab-org/gitlab-ce/issues/56557. Renames cluster#managed? method to cluster#provided_by_user?. At the moment managed? is being used to referring those clusters that were manually added through the "Add existing cluster" option.

This change will allow us to use managed? on https://gitlab.com/gitlab-org/gitlab-ce/issues/56557 for referring clusters that are going to be managed by GitLab.

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