Update Rust crate serde to v1.0.209

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
serde (source)	dependencies	patch	1.0.204 -> 1.0.209

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Release Notes

serde-rs/serde (serde)

v1.0.209

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• Fix deserialization of empty structs and empty tuples inside of untagged enums (#​2805, thanks @​Mingun)

v1.0.208

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• Support serializing and deserializing unit structs in a flatten field (#​2802, thanks @​jonhoo)

v1.0.207

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• Improve interactions between flatten attribute and skip_serializing/skip_deserializing (#​2795, thanks @​Mingun)

v1.0.206

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• Improve support for flatten attribute inside of enums (#​2567, thanks @​Mingun)

v1.0.205

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• Use serialize_entry instead of serialize_key + serialize_value when serialize flattened newtype enum variants (#​2785, thanks @​Mingun)
• Avoid triggering a collection_is_never_read lint in the deserialization of enums containing flattened fields (#​2791)

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