chore(algebra/group_power/lemmas): Lemmas about gsmul (#8618)
This restates some existing lemmas as `monotone` and `strict_monotone`, and provides new lemmas about the right argument of gsmul:
* `gsmul_le_gsmul'`
* `gsmul_lt_gsmul'`
* `gsmul_le_gsmul_iff'`
* `gsmul_lt_gsmul_iff'`
This also removes an unnecessary `linear_order` assumption from `gsmul_le_gsmul_iff` and `gsmul_lt_gsmul_iff`.
Co-authored-by: Eric Wieser <wieser.eric@gmail.com>
