Issue #3042223 follow-up by alexpott: Map text_plain field formatter to string

(cherry picked from commit e1f5186b43f94d829cac154476dd6708d34f1f48)

core/modules/text/tests/src/Unit/Plugin/migrate/field/d7/TextFieldTest.php

@@ -39,7 +39,7 @@ public function getFieldFormatterTypeProvider() {
    * @covers ::getFieldType
    * @dataProvider getFieldFormatterTypeProvider
    */
-  public function testGetFieldType(string $type, string $formatter_type, string $expected) {
+  public function testGetFieldType($type, $formatter_type, $expected) {
     $row = new Row();
     $row->setSourceProperty('type', $type);
     $row->setSourceProperty('formatter/type', $formatter_type);