Update Rust crate serde_with to v3.21.0

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
serde_with	dependencies	minor	3.20.0 → 3.21.0

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Release Notes

jonasbb/serde_with (serde_with)

v3.21.0: serde_with v3.21.0

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Security

• GHSA-7gcf-g7xr-8hxj: KeyValueMap serialization panics on empty sequence or map entries
  Bad or attacker controlled values could cause a panic while allocating too large values.
  Fixed in #​966 by setting a maximum allocation size during the creation of collections like Vec or sets.

  Thanks to @​7thParkk for reporting the issue.

Added

• Add NoneAsZero adapter that maps Option<NonZero*> to a plain integer, encoding None as 0 by @​SAY-5 (#​486)

Changed

• Re-enable link-to-definition on docs.rs (#​964)

Fixed

• Fix some doc links to point to the correct types (#​963)
• Re-enable unused_qualifications and fix the resulting findings by @​lms0806 (#​962)

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