Release 0.3.28

[taiki-e]

Changes:

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

Backports:

• https://github.com/rust-lang/futures-rs/pull/2580
• https://github.com/rust-lang/futures-rs/pull/2726
• https://github.com/rust-lang/futures-rs/pull/2723
• https://github.com/rust-lang/futures-rs/pull/2728
• https://github.com/rust-lang/futures-rs/pull/2729
• https://github.com/rust-lang/futures-rs/pull/2732
• https://github.com/rust-lang/futures-rs/pull/2733
• https://github.com/rust-lang/futures-rs/pull/2730