chore: fix no_std builds

I’m not entirely sure if it’s no_std specifically that causes the
issue, but if ibc-apps-transfer is build without std feature but with
serde feature it enables serde_json/std feature while serde_json
dependency isn’t enabled.  This seems to confuse Cargo which thinks
serde_json is supposed to be a feature.  This all leads to a failure
in dependency resolution:

    error: failed to select a version for `ibc-app-transfer`.
        ... required by package `ibc-apps v0.48.0`
        ... which satisfies dependency `ibc-apps = "^0.48.0"` (locked to 0.48.0) of package `ibc v0.48.0`
        ... which satisfies dependency `ibc = "^0.48.0"` (locked to 0.48.0) of package `ibc-testkit v0.48.0`
        ... which satisfies dependency `ibc-testkit = "^0.48.0"` (locked to 0.48.0) of package …
    versions that meet the requirements `^0.48.0` (locked to 0.48.0) are: 0.48.0

    the package `ibc-apps` depends on `ibc-app-transfer`, with features: `serde_json` but `ibc-app-transfer` does not have these features.
     It has an optional dependency with that name, but that dependency uses the "dep:" syntax in the features table, so it does not have an implicit feature with that name.

.changelog/unreleased/bug-fixes/987-serde-json-feature.md

@@ -0,0 +1,2 @@
+- Fix dependency resolution whan building no_std
+  ([#987](https://github.com/cosmos/ibc-rs/issues/987))

ibc-apps/ics20-transfer/Cargo.toml

@@ -33,7 +33,7 @@ default = ["std"]
 std = [
     "ibc-app-transfer-types/std",
     "ibc-core/std",
-    "serde_json/std",
+    "serde_json?/std",
 ]
 serde = [
     "ibc-app-transfer-types/serde",