chore: switch test from bincode to maintained postcard crate (RUSTSEC-2025-0141 ) (#9104)

# Which issue does this PR close?

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- Closes #9009
- Also addresses https://rustsec.org/advisories/RUSTSEC-2025-0141 --
this is only used for testing but still

# Rationale for this change

https://crates.io/crates/bincode is unmaintained

<img width="833" height="657" alt="Screenshot 2026-01-06 at 5 57 13 PM"
src="https://github.com/user-attachments/assets/9e18a6e5-eb45-4470-94b7-e17b89ad558e"
/>

There also appears to be some sort of drama related to the maintainer

While we only use this code in tests, it would be nice to avoid issues
sooner rather than later

# What changes are included in this PR?

Change to use postcard

# Are these changes tested?
by ci

# Are there any user-facing changes?

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Author: alamb

arrow-schema/Cargo.toml

@@ -53,12 +53,9 @@ serde = ["dep:serde_core", "dep:serde"]
 all-features = true
 
 [dev-dependencies]
-bincode = { version = "2.0.1", default-features = false, features = [
-    "std",
-    "serde",
-] }
 criterion = { workspace = true, default-features = false }
 insta = "1.43.1"
+postcard = { version = "1.0.10", default-features = false, features = ["use-std"] }
 
 [[bench]]
 name = "ffi"

arrow-schema/src/field.rs

@@ -1448,10 +1448,8 @@ mod test {
 
     #[cfg(feature = "serde")]
     fn assert_binary_serde_round_trip(field: Field) {
-        let config = bincode::config::legacy();
-        let serialized = bincode::serde::encode_to_vec(&field, config).unwrap();
-        let (deserialized, _): (Field, _) =
-            bincode::serde::decode_from_slice(&serialized, config).unwrap();
+        let serialized = postcard::to_stdvec(&field).unwrap();
+        let deserialized: Field = postcard::from_bytes(&serialized).unwrap();
         assert_eq!(field, deserialized)
     }