Update import group screen to specify *Import*

What does this MR do?

The current "Import Group" tab, shown here, suggests that it's used for "GitLab group export":

That UI text is IMO misleading, as the requirement is to import the tar.gz zipped file that was exported from a different GitLab instance.

We therefore need to change this text from "GitLab group export" to "GitLab group import".

Problem discovered while discussing #241606 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team