Change: #959 ditch async_trait for the singlethreade feature

[wvwwvwwv]

Issue #959.

This PR substitutes the existing async_trait macro usage with the async_fn_in_trait language feature which is generally available in Rust 1.75.0 if the singlethreaded feature is turned on.

With this change, in order to use the singlethreaded feature, the minimum supported Rust version is bumped to 1.75.0.

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[wvwwvwwv]

@schreter

[drmingdrmer]

Thank you! It looks good to me

Reviewed 2 of 3 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @schreter)

[schreter]

Ah, the change is much smaller than I expected, nice :-).

Reviewed 2 of 3 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @wvwwvwwv)