Replace `slim/psr7` with `nyholm/psr7`.

composer.json

composer.lock

lib/bootstrap-definitions.php

lib/classes/JsonApi/JsonApiController.php

lib/classes/JsonApi/JsonApiIntegration/Responses.php

lib/classes/JsonApi/Middlewares/Authentication.php

lib/classes/JsonApi/Middlewares/DangerousRouteHandler.php

lib/classes/JsonApi/Middlewares/RemoveTrailingSlashes.php

lib/classes/JsonApi/Routes/Avatar/AvatarUpload.php

lib/classes/JsonApi/Routes/Files/FileRefsContentShow.php

lib/classes/JsonApi/Routes/Files/RoutesHelperTrait.php

lib/classes/JsonApi/Routes/Holidays/HolidaysShow.php

lib/classes/JsonApi/Routes/StockImages/StockImagesUpload.php

lib/classes/JsonApi/dependencies.php

lib/classes/JsonApi/middleware.php

lib/classes/JsonApi/routes.php

lib/classes/LtiLink.php

lib/classes/OAuth2/NegotiatesWithPsr7.php

tests/_support/Helper/Jsonapi.php

tests/_support/_generated/FunctionalTesterActions.php