Update approve all pending members confirm wording

What does this MR do and why?

It updates the wording of the "Approve All" modal.

Note: no changelog is added since the changes are behind a disabled feature flag.

Screenshots or screen recordings

When user cap is not set

singular	plural

When user cap is set

singular	plural

How to set up and validate locally

1. Set self.com? to return true by adding return true after line 58 in lib/gitlab.rb
2. Enable saas_user_caps feature flag from rails console. (rails console then Feature.enable(:saas_user_caps))
3. With a group with an Ultimate plan
4. Navigate to Settings -> Permissions and group features -> User Cap
5. Set the user cap limit to 1
6. Navigate to the group members
7. Add one or more members (until you are above the user cap limit set in previous step)
8. Navigate to Usage Quota from the left sidebar under Settings
9. You should see an alert that you have pending members
10. Clicking the button in the alert should navigate to a pending members page
11. Observe the "Approve All" button at the top right, clicking it will show a confirmation modal
12. After confirming the pending members now should be moved from the invited tab to the members tab in the group's members page

Note: to test the case where the user cap is not set, make sure you followed the steps above, keep a few pending members, then remove the user cap (like step 4, but remove the value from the user cap limit field).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #390296 (closed)