Updating UI from admins to administrators

What does this MR do and why?

This MR fixes the changes requested by the technical writer in the following thread: !166456 (comment 2130025323)

We're out of sync between the tech docs and the UI - which has caused confusions in product translation. In this doc/administration/settings/import_and_export_settings.md file. we've updated the wording from admins to administrators. I'm making a quick fix to the error message and relevant tests to update accordingly.

Error message:

Before	After
You can assign only active users with regular or auditor access. To assign users with administrator access, ask your GitLab administrator to enable the \"Allow contribution mapping to admins\" setting.	You can assign only active users with regular or auditor access. To assign users with administrator access, ask your GitLab administrator to enable the \"Allow contribution mapping to administrators\" setting.

References

Originating issue: #474963 (closed)

and the Merge Request: !166456 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests. <Require assistance on creating the error message in my local>

Before	After
	You can assign only active users with regular or auditor access. To assign users with administrator access, ask your GitLab administrator to enable the \"Allow contribution mapping to administrators\" setting.

How to set up and validate locally

The complete steps can be found in the original MR: !166456 (merged)

More information: https://docs.gitlab.com/ee/administration/settings/import_and_export_settings.html#allow-contribution-mapping-to-administrators