net: lwm2m: replace deprecated lwm2m_engine_get_resource

`lwm2m_engine_get_resource()` has been mark for deprecation for
more than 2 releases, but we are still using it in tree now,
because the `__deprecated` marker was probably forgotten in the
initial deprecation commit.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>