Fix missing close paren in crates-io.md

    warning: the feature `error_in_core` has been stable since 1.81.0-nightly and no longer requires an attribute to enable
       --> serde/src/lib.rs:108:43
        |
    108 | #![cfg_attr(feature = "unstable", feature(error_in_core, never_type))]
        |                                           ^^^^^^^^^^^^^
        |
        = note: `#[warn(stable_features)]` on by default

    error: struct `RemoteSelf` is never constructed
       --> test_suite/tests/test_gen.rs:425:12
        |
    425 |     struct RemoteSelf;
        |            ^^^^^^^^^^
        |
    note: the lint level is defined here
       --> test_suite/tests/test_gen.rs:5:9
        |
    5   | #![deny(warnings)]
        |         ^^^^^^^^
        = note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`

This change enables the `#[diagnostic::on_unimplemented]` attribute for
the `Serialize` and `Deserialize` trait to point the user to the
relevant derives and point out that they might want to check crates
features for external types by adding the relevant hints an note.

…ented

Use the `#[diagnostic::on_unimplemented]` attribute when possible

The test suite's dependencies cannot be resolved by an old toolchain.

    error: failed to select a version for the requirement `toml = "^0.8"`
    candidate versions found which didn't match: 0.5.11, 0.5.10, 0.5.9, ...
    location searched: crates.io index
    required by package `trybuild v1.0.97`
        ... which satisfies dependency `trybuild = "^1.0.97"` of package `serde_test_suite v0.0.0`

Add ui test of unsatisfied serde trait bound