1.51 support broken due to async-global-executor upgrade

[djc]

2.1.0 requires 1.59. Some discussion in async-rs/async-global-executor#10.

[jaymell]

What if we were to as a short-term measure pin the async-std version as follows:

async-std = { version = "= 1.8.0", optional = true}

[djc]

Yeah, I guess that's reasonable, although we won't want to release with that.

[nihohit]

@djc is this still relevant?

[djc]

Given that the current MSRV is 1.65.0, I guess not.