Issue #1802128 by legolasbo, jucallme: Fixed Replace "user name" with "username" in UI text.

core/modules/basic_auth/basic_auth.module

core/modules/contact/contact.module

core/modules/migrate_drupal/src/Tests/Dump/Drupal6User.php

core/modules/search/search.module

core/modules/system/src/Tests/Entity/EntityFieldTest.php

core/modules/system/src/Tests/Form/ArbitraryRebuildTest.php

core/modules/user/src/AccountForm.php

.../user/src/Plugin/Validation/Constraint/UserNameUnique.php

core/modules/user/src/Tests/UserCancelTest.php

core/modules/user/src/Tests/UserEditTest.php

core/modules/user/src/Tests/UserSearchTest.php

core/modules/user/src/Tests/UserValidationTest.php

core/modules/user/user.module