try-runtime: run migration checks per default (#13684)

* try-runtime: run migration checks per default

The current behaviour of having to explicetly specify --checks seems
to cause confusion. Therefore bringing back the old behaviour of
running the pre- and post-upgrade checks per default.

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Fix docs

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>