Bump MSRV of core crates to 1.56.0

[brody4hire]

STATUS:

I think the following PRs should be merged first to resolve the CI build issues:

• resolve unnecessary map_or Clippy issue #2904 - clippy failure
• Bump general MSRV to 1.63 #2907 - bump main / general MSRV to 1.63

I would be happy to rebase & squash with a clean commit message once the PRs above are reviewed & merged.

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WHY:

This should allow the following updates I proposed in other PRs:

• use core::panic::AssertUnwindSafe rather than depending on std for this in futures-core - other PR core: no-std panic::AssertUnwindSafe (etc.) #2908
• update Rust edition -> 2021 - other PR update Rust edition -> 2021 #2909

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open question(s):

• This PR proposed exact core MSRV, while other PR proposes less exact general MSRV - see Bump general MSRV to 1.63 #2907 - I would be happy to change either of these to improve consistency if needed. I made this one more exact since there is also Rust version 1.56.1.
• I wonder if we should just use the same MSRV in all crates to keep things a little simpler. I would be happy to close this and update the MSRV for all crates in PR Bump general MSRV to 1.63 #2907.

[taiki-e]

Closing as #2908 and #2909 are closed.

Thanks anyway for the PR.