Issue #3443946 by pfrenssen, kksandr, akulsaxena: Address warnings reported in Gitlab CI

.gitlab-ci.yml

src/Event/WebformSubmitReturnEvent.php

src/Form/WebformRestForm.php

...lugin/rest/resource/WebformCompleteSubmissionResource.php

src/Plugin/rest/resource/WebformElementsResource.php

src/Plugin/rest/resource/WebformFieldsResource.php

src/Plugin/rest/resource/WebformSubmissionResource.php

src/Plugin/rest/resource/WebformSubmitResource.php

tests/src/Functional/WebformRestCompleteSubmissionTest.php

tests/src/Functional/WebformRestElementsTest.php

tests/src/Functional/WebformRestFieldsTest.php

tests/src/Functional/WebformRestSubmissionTest.php

webform_rest.install