Warn about deprecated process usage

[soxofaan]

Related to the version issue descsribed at #240

Can we more clearly communicate deprecation warnings, e.g. when user is using deprecated/experimental processes, including tips on what should be used instead?

For batch jobs we could use batch job logs (at risk of getting lost in the large volume of logs).
For sync execution, there is not really a warning/logging channel.

At Open-EO/openeo-api#412 I already tried to push for a warnings channel outside of batch job contexts, but nothing significant landed there yet

[soxofaan]

For sync (and batch jobs), we could also leverage the validation feature (which is now enabled by default from python client) to list usage of certain deprecated/experimental processes