Rename PageVariantEntity back to PageVariant.

src/Controller/PageManagerController.php

src/Entity/Page.php

src/Entity/PageVariant.php

src/Form/PageEditForm.php

src/Form/PageVariantAddForm.php

src/Form/PageVariantConfigureForm.php

src/Form/PageVariantDeleteForm.php

src/Form/PageVariantEditForm.php

src/Form/PageVariantFormBase.php

src/Form/SelectionConditionDeleteForm.php

src/Form/SelectionConditionFormBase.php

src/Form/VariantPluginAddBlockForm.php

src/Form/VariantPluginConfigureBlockFormBase.php

src/Form/VariantPluginDeleteBlockForm.php

src/PageExecutable.php

src/PageExecutableInterface.php

src/PageInterface.php

src/PageVariantInterface.php

src/Tests/PageManagerAdminTest.php

tests/src/Unit/PageExecutableTest.php