Bump serde from 1.0.203 to 1.0.204

Bumps serde from 1.0.203 to 1.0.204.

Release notes

Sourced from serde's releases.

v1.0.204

• Apply #[diagnostic::on_unimplemented] attribute on Rust 1.78+ to suggest adding serde derive or enabling a "serde" feature flag in dependencies (#2767, thanks @​weiznich)

Commits

• 18dcae0 Release 1.0.204
• 58c307f Alphabetize list of rustc-check-cfg
• 8cc4809 Merge pull request #2769 from dtolnay/onunimpl
• 1179158 Update ui test with diagnostic::on_unimplemented from PR 2767
• 91aa40e Add ui test of unsatisfied serde trait bound
• 595019e Cut test_suite from workspace members in old toolchain CI jobs
• b0d7917 Pull in trybuild 'following types implement trait' fix
• 8e6637a Merge pull request #2767 from weiznich/feature/diagnostic_on_unimplemented
• 694fe05 Use the #[diagnostic::on_unimplemented] attribute when possible
• f3dfd2a Suppress dead code warning in test of unit struct remote derive
• Additional commits viewable in compare view

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