Display pending user count when modifying user cap

What does this MR do?

This MR makes use of the pending user count added in !66619 (merged), to display the number of pending users that may be auto-approved if an instance admin modifies the signup user cap accordingly (increases or removes cap).

You may notice a slight deviation from the UX design has been made - the modal title has dropped the word all, as per Amanda's comment here: #331554[Frame_1.png]

This MR also adjusts the logic used to display the modal, to only display it if there are pending members that maybe auto-approved (also mentioned in the linked design issue above)

Refs #328237 (closed)

Screenshots or Screencasts (strongly suggested)

Design:

UX Issue

single pending user	multiple pending users

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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