Fix: Translation loading for the wp-user-manager domain was triggered too early.

[reygcalantaol]

Resolves #405 and https://github.com/WPUserManager/wpum-custom-fields/issues/88

Description

Since WordPress 6.7, calling translation functions before the init hook will trigger a PHP notice.

Testing Instructions

1. Enable WordPress DEBUG
2. Review the debug.log file and look for PHP Notice: Function _load_textdomain_just_in_time was called entry.

Pre-review Checklist

• Issue and pull request titles are properly formatted.
• Acceptance criteria have been satisfied and marked in the related issue.
• Unit tests are included (if applicable).
• Self-review of code changes has been completed.
• Self-review of UX changes has been completed.
• Review has been requested from @polevaultweb.