embedded-nal-async half-needlessly requires nightly

[chrysn]

As both the embedded-nal-async versions that are supported are behind the same flag, users of the 0.7 version (that works with stable) still pull in 0.6 (that requires nightly).

Options are:

• ditch 0.6 in a 0.8.3 (because after all, it requires nightly so it's not really a breaking change ... yeah well it is but it wasn't in for long ... which also doesn't make it less of a breaking change)
• add a gate that is not enabling 0.6 but enables 0.7 ... or remove the gate for 0.7 altogether (just like other gates were removed in the past)
• just do the next breaking version already and ditch 0.6 there (there's so many tickets right now that do wait for a breaking version)

[chrysn]

With #90 merged, 0.6 was ditched.