6 files declare their use of LayoutInterface in Drupal 9

ConfigureSectionForm.php in core/modules/layout_builder/src/Form/ConfigureSectionForm.php

EntityDisplayWithLayoutInterface.php in core/modules/field_layout/src/Display/EntityDisplayWithLayoutInterface.php

FieldLayoutEntityDisplayTrait.php in core/modules/field_layout/src/Entity/FieldLayoutEntityDisplayTrait.php

SectionComponentTest.php in core/modules/layout_builder/tests/src/Unit/SectionComponentTest.php

SectionRenderTest.php in core/modules/layout_builder/tests/src/Unit/SectionRenderTest.php

SectionTest.php in core/modules/layout_builder/tests/src/Unit/SectionTest.php