Update workspace GraphQL clusterAgent filter ENUM name to reflect usage
MR: Pending
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## Description
Follow-up from https://gitlab.com/gitlab-org/gitlab/-/merge_requests/192329#note_2522698873+
Given that the filter enum values are shared across `namespace` and `organization`, it would be proper to make the enum name and doc more generic to reflect its usage.
Proposed name: `ClusterAgentFilterEnum`
Note: we must make sure that we are not causing any breaking changes with this name change (APIs affected should be experimental to allow for leeway)
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