Bump serde from 1.0.208 to 1.0.209 in /rust

[dependabot[bot]]

Bumps serde from 1.0.208 to 1.0.209.

Release notes

Sourced from serde's releases.

v1.0.209

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

Commits

• 30752ac Release 1.0.209
• b84e6ca Improve wording of PR 2805 comments
• 87a2fb0 Wrap comments from PR 2805 to 80 columns
• 9eaf7b9 Merge pull request #2805 from Mingun/untagged-tests
• 7bde100 Replace MapRefDeserializer with value::MapDeserializer
• da7fc79 Fix deserialization of empty struct variant in untagged enums
• 4c5fec1 Test special cases that reaches SeqRefDeserializer::deserialize_any len==0 co...
• 6588b0a Cover Content::Seq case in VariantRefDeserializer::struct_variant
• 0093f74 Split test newtype_enum into four tests for each variant
• 171c6da Complete coverage of ContentRefDeserializer::deserialize_newtype_struct
• Additional commits viewable in compare view

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