Bump proptest from 1.5.0 to 1.6.0 (#6067)

Bumps [proptest](https://github.com/proptest-rs/proptest) from 1.5.0 to
1.6.0.
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rs/backend/Cargo.toml

@@ -48,7 +48,7 @@ candid_parser = "0.1.2"
 ic_principal = "0.1.0"
 maplit = "1.0.2"
 pretty_assertions = "1.4.0"
-proptest = "1.5.0"
+proptest = "1.6.0"
 rand = "0.8.5"
 slotmap = "1"
 tokio = { version = "1.39.3", features = ["full"] }