Add unit-level matrix specs for newly-introduced policies

MR: Add ability and matrix policy specs (!150190 - merged)

Description

In Add mutation APIs for group-agent authorizatio... (!145387 - merged), we forgot to add a thorough matrix-based policy unit test for the newly introduced policies.

For example, we have a thorough matrix test in ee/spec/policies/remote_development/workspace_policy_spec.rb for all roles: https://gitlab.com/gitlab-org/gitlab/-/blob/fa08c704df0e829e3d0131725e588d07b011d9c9/ee/spec/policies/remote_development/workspace_policy_spec.rb

We should have similar policy unit specs for the following files:

• ee/app/policies/remote_development/agent_policy.rb
• ee/app/policies/remote_development/group_policy.rb

Also, per the discussion in this thread, we will add the following policy and related specs for ee/app/policies/remote_development/group_policy.rb, as part of this MR:

rule { can?(:maintainer_access) | can?(:owner_access) }.enable :read_remote_development_cluster_agent_mapping

We will also clean up some of the existing Remote Development policies to switch owner to can?(:owner_access) and remove unnecessary admin.

See also Discussion: modularisation in policies (#454934 - closed)

Acceptance Criteria

• Add read_remote_development_cluster_agent_mapping policy
• Clean up ee/spec/policies/remote_development/workspace_policy_spec.rb
• Remove unnecessary 'admin' in ee/app/policies/remote_development/agent_policy.rb
• Backfill matrix unit specs for ee/app/policies/remote_development/agent_policy.rb
• Switch owner to can?(:owner_access) and remove unnecessary 'admin' in ee/app/policies/remote_development/group_policy.rb
• Backfill matrix unit specs for ee/app/policies/remote_development/group_policy.rb