Update workspace GraphQL clusterAgent filter ENUM name to reflect usage

MR: Pending

Description

Follow-up from !192329 (comment 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)

Acceptance criteria

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Implementation plan

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