Removing FilesSearch feature flag (#17548)

* Removing FilesSearch feature flag

* Fixing tests

* Adding an improvement on plain text extraction

* Adding tests for plain text extraction

* Removed unneeded conversion

* Adding missed license

* Remove the feature flag from the migration

* Fixing some tests

* Updating i18n/en.json file

testlib/store.go

@@ -24,6 +24,7 @@ func (s *TestStore) Close() {
 func GetMockStoreForSetupFunctions() *mocks.Store {
 	mockStore := mocks.Store{}
 	systemStore := mocks.SystemStore{}
+	systemStore.On("GetByName", "ContentExtractionConfigDefaultTrueMigrationComplete").Return(&model.System{Name: "ContentExtractionConfigDefaultTrueMigrationComplete", Value: "true"}, nil)
 	systemStore.On("GetByName", "UpgradedFromTE").Return(nil, model.NewAppError("FakeError", "app.system.get_by_name.app_error", nil, "", http.StatusInternalServerError))
 	systemStore.On("GetByName", "ContentExtractionConfigMigrationComplete").Return(&model.System{Name: "ContentExtractionConfigMigrationComplete", Value: "true"}, nil)
 	systemStore.On("GetByName", "AsymmetricSigningKey").Return(nil, model.NewAppError("FakeError", "app.system.get_by_name.app_error", nil, "", http.StatusInternalServerError))