API: Deprecate ordered=False and legacy some functions

[larsoner]

Closes #11546
Closes #11531

I think this is probably good enough to close #11531. @drammock you had slight differences / a bit more stuff in #11531 (comment) . For example, I left out the new forward instance method as I think picking channels for forward is very rare for users so the cost of having to use pick_channels_forward is low (same with cov), and hence not worth the code churn to replace it. I did add an ordered=None to those functions, though, in order to minimize the risk of problems for people, which I think is the most critical thing end-user-safety-wise.

I also did not @legacy the mne.pick_types function for now because 1) there is no straightforward replacement that allows you to get the indices of given channel types, and 2) we would have to alias all of our internal uses to a private _pick_types to avoid the legacy print (and there are a lot of them). I did @legacy the instance method inst.pick_types, though, which seems reasonable. Same goes for mne.pick_channels and inst.pick_channels.

If/when this is reviewed and approved and I've fixed all the tests (I'm sure some will have broken!) I'll push a [circle full] before merge so we can find out now about any broken examples/tutorials rather than this evening.

[drammock]

just a few comments, thanks for tackling this @larsoner

[larsoner]

Okay ran the [circle full] and modified the examples that failed. Two approvals so I'm marking for merge-when-green!