Update username validator

[sainak]

This pull request updates the username validator in the User model. The new validator ensures that the username meets the required criteria, including length, allowed characters, and consecutive characters. It also includes unit tests to validate the validator's behavior.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (553b2a5) 61.66% compared to head (9515dfb) 61.67%.
Report is 10 commits behind head on master.

❗ Current head 9515dfb differs from pull request most recent head 4e8a369. Consider uploading reports for the commit 4e8a369 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1865      +/-   ##
==========================================
+ Coverage   61.66%   61.67%   +0.01%     
==========================================
  Files         212      212              
  Lines       11691    11695       +4     
  Branches     1658     1659       +1     
==========================================
+ Hits         7209     7213       +4     
  Misses       4184     4184              
  Partials      298      298              

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