feat: add group selection to invite members modal

[katrinan029]

Description

Updates the members invite modal to include a dropdown menu with flex groups.
https://2u-internal.atlassian.net/browse/ENT-9490

UI Changes

Testing

1. checkout branch knguyen2/ent-9490 and visit link https://localhost.stage.edx.org:1991/exec-ed-2u-integration-qa/admin/learner-credit/e259a8e5-e0ad-4c9d-bf14-9abf310158c7
2. confirm that you see a dropdown menu
3. select a group and confirm that the email populates in the summary section

For all changes

• Ensure adequate tests are in place (or reviewed existing tests cover changes)

Only if submitting a visual change

• Ensure to attach screenshots
• Ensure to have UX team confirm screenshots

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.78%. Comparing base (4e72019) to head (a00966e).

Files with missing lines	Patch %	Lines
...nents/PeopleManagement/CreateGroupModalContent.jsx	89.18%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1344      +/-   ##
==========================================
+ Coverage   85.73%   85.78%   +0.05%     
==========================================
  Files         597      598       +1     
  Lines       13071    13119      +48     
  Branches     2766     2768       +2     
==========================================
+ Hits        11206    11254      +48     
+ Misses       1800     1799       -1     
- Partials       65       66       +1     

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