Update Rust crate futures to 0.3.28

This MR contains the following updates:

Package	Type	Update	Change
futures (source)	dependencies	patch	0.3.27 -> 0.3.28
futures (source)	dependencies	patch	0.3.26 -> 0.3.28

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Release Notes

rust-lang/futures-rs (futures)

v0.3.28

Compare Source

• Update to syn 2. This raises MSRV of utility crates to 1.56. (#​2730, #​2733)
• Fix bug in FlattenUnordered (#​2726, #​2728)

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