[SegFormer] Add deprecation warning
# What does this PR do?

This PR takes it a bit more light compared to #15748. Rather than removing the `reshape_last_stage` argument right away, this PR instead tells users that this argument is deprecated and will soon be removed.

After some weeks, we can then merge #15748.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15889). All of your documentation changes will be reflected on that endpoint.