update Rust edition -> 2021

[brody4hire]

requires PR #2907 - bump general MSRV to 1.63; need to update core MSRV as stated below

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CI testing TODO(s):

• clippy failure - as already proposed by other PR: resolve unnecessary map_or Clippy issue #2904
• core MSRV failure - requires core MSRV bump to 1.41 - as already proposed by other PR: Bump MSRV of core crates to 1.56.0 #2906
• expected general MSRV failure - requires general MSRV -> 1.63 - as already proposed by other PR: Bump general MSRV to 1.63 #2907

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other TODO(s):

• update workspace resolver -> "2", as stated by cargo warning
• update core MSRV -> 1.56 in other PR & merge once ready: Bump MSRV of core crates to 1.56.0 #2906
• update main MSRV -> 1.63 in other PR: Bump general MSRV to 1.63 #2907

[brody4hire]

https://github.com/rust-lang/futures-rs/actions/runs/12644300502/job/35231663009?pr=2909 - it looks to me like we would need to update core MSRV to something more recent than 1.41 to work with this update

[taiki-e]

I'm going to close this because I'm not interested in increasing MSRV just for the editions, but thanks anyway for the PR.