Issue #2354597 by esod, YesCT, vadim.hirbu, ashutoshsngh: Remove usage of form_set_value().

...Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php

core/modules/config/src/Form/ConfigSingleImportForm.php

core/modules/entity_reference/entity_reference.module

...e/src/Plugin/Field/FieldWidget/AutocompleteTagsWidget.php

...rence/src/Plugin/Field/FieldWidget/AutocompleteWidget.php

.../src/Plugin/Field/FieldWidget/TestFieldWidgetMultiple.php

core/modules/field_ui/src/FieldOverview.php

core/modules/field_ui/src/Form/EntityDisplayModeAddForm.php

core/modules/file/file.module

core/modules/file/src/Plugin/Field/FieldType/FileItem.php

core/modules/filter/src/FilterFormatFormBase.php

core/modules/image/src/Plugin/Field/FieldType/ImageItem.php

core/modules/node/node.api.php

core/modules/system/src/Form/SiteInformationForm.php

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

.../modules/system/tests/modules/form_test/src/Callbacks.php

...tests/modules/form_test/src/Form/FormTestValidateForm.php

core/modules/taxonomy/taxonomy.module

core/modules/user/src/Form/UserPasswordForm.php

.../src/Plugin/entity_reference/selection/ViewsSelection.php