fix: update autogenerated username functionality

[attiyaIshaque]

Description

"When entering special characters like '=', '@', or any other, it raised a server error when the autogenerated username was enabled. I have fixed it in the backend, and now users are allowed to create accounts with special characters." Useful information to include:

JIRA:

https://2u-internal.atlassian.net/browse/VAN-1988

How Has This Been Tested?

Updated unit tests

Video :

https://github.com/openedx/edx-platform/assets/7627421/abdeded3-f029-481d-8ed5-2fcf015d3726

[attiyaIshaque]

From where was the exception being raised? What will be the generated username if someone passes = as a name?

The username will be as expected

f"{current_year}{current_month}_{random_string}"

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.