migrate new benchmarking syntax from `frame_support::benchmarking` to...

migrate new benchmarking syntax from `frame_support::benchmarking` to `frame_benchmarking::v2` (#13235)

* * re-export frame_support::benchmarking in frame_benchmarking::
* prefer use frame_benchmarking::*; in examples, etc

* switch to frame_benchmarking::v2

* completely migrate new benchmarking code out of frame_support

* fix doc links

* remove unneeded return

Co-authored-by: Bastian Köcher <git@kchr.de>

* remove another unneeded return

Co-authored-by: Bastian Köcher <git@kchr.de>

* properly export all macros in v1

* refactor existing frame_benchmarking imports to use ::v1

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Co-authored-by: Bastian Köcher <git@kchr.de>