[FIX] User merge by e-mail on OAuth is case-sensitive

[matheusbsilva137]

Proposed changes (including videos or screenshots)

• Ignore case when searching for a user by email.

Issue(s)

Fixes #22832

Steps to test or reproduce

1. Manually register a user with an e-mail containing upper-case letters (eg Joe.Test@example.com);
2. Set up an OAuth server, enable the "Merge users" option and choose "Email" as the "Key Field";
3. Using OAuth, try to login with a user with the same e-mail registered previously, but all in lower-case letter (eg joe.test@example.com`);

Further comments

TC-80

[codecov]

Codecov Report

Merging #27167 (5ebcdf3) into develop (91844b2) will increase coverage by 0.63%.
The diff coverage is 67.56%.

@@             Coverage Diff             @@
##           develop   #27167      +/-   ##
===========================================
+ Coverage    41.02%   41.66%   +0.63%     
===========================================
  Files          833      826       -7     
  Lines        18407    18234     -173     
  Branches      2047     1987      -60     
===========================================
+ Hits          7552     7597      +45     
+ Misses       10568    10351     -217     
+ Partials       287      286       -1     

Flag	Coverage Δ
e2e	41.66% <67.56%> (+0.63%)	⬆️

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