Use `FutureWarning` instead of `DeprecationWarning` for deprecation warning

basically a copy of https://github.com/PyTorchLightning/metrics/issues/744 which was addressed in https://github.com/PyTorchLightning/metrics/pull/749

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🚀 Feature

see suggestion in https://github.com/PyTorchLightning/metrics/pull/740#discussion_r782088021

Motivation

most of the deprecations in TM are meant to users not developers

Pitch

Replace DeprecationWarning with FutureWarning defined at: https://github.com/PyTorchLightning/pytorch-lightning/blob/033dba1494a177954e8ca59bc74b1635e83b9efa/pytorch_lightning/utilities/warnings.py#L44 and remove: https://github.com/PyTorchLightning/pytorch-lightning/blob/033dba1494a177954e8ca59bc74b1635e83b9efa/pytorch_lightning/utilities/warnings.py#L48-L49

Alternatives

Keep using DeprecationWarning.

Additional context

• exception DeprecationWarning Base class for warnings about deprecated features when those warnings are intended for other Python developers. Ignored by the default warning filters, except in the __main__ module (PEP 565). Enabling the Python Development Mode shows this warning.

• exception FutureWarning Base class for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.

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cc @borda