fiber: make fiber_set_cancellable a no-op

Currently this function is not used inside Tarantool, however it is
available via C module API. Deprecate it, because it is very confusing
and has nothing to do with the fiber cancellation.

Closes #7166

@TarantoolBot document
Title: fiber: get rid of fiber_set_cancellable
Product: Tarantool
Since: 2.11
Audience/target: dev
Root document: https://www.tarantool.io/en/doc/latest/dev_guide/reference_capi/fiber/#c.fiber_set_cancellable
SME: @Gumix

This function is a no-op since 2.11 and should be dropped from the
documentation.