[SaaS] User cap for groups settings text should be SaaS-specific

What does this MR do and why?

This change updates the help text for user access controls in group settings. The wording is being changed from "administrator" to "group owner" to more accurately reflect who has the authority to approve user access requests. Specifically, it clarifies that when a user cap is set, any users added beyond that limit need approval from a group owner (not an administrator), and when open access is enabled, invitations don't require group owner approval. This is primarily a terminology correction to make the interface more precise about permission levels.

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Related to #427732 (closed)