Bump Kotlin and Dokka versions to latest release

[kggilmer]

Issue #

Companion PRs:

• Bump Kotlin version to latest release awslabs/aws-crt-kotlin#37
• Bump Kotlin and Dokka versions to latest release awslabs/aws-sdk-kotlin#360

Description of changes

• Update Kotlin and Dokka to 1.5.31 to support documentation generation

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ianbotsf]

Question: While the gradle.properties changes seem fine, I see the version number listed in other places:

• runtime/runtime-core/build.gradle.kts
• runtime/serde/serde-json/TESTING.md
• KotlinDependency.kt

Do these locations need to be updated as well?

[aajtodd]

Question: While the gradle.properties changes seem fine, I see the version number listed in other places:

* [**runtime/runtime-core/build.gradle.kts**](https://github.com/awslabs/smithy-kotlin/blob/main/runtime/runtime-core/build.gradle.kts#L7)

* [**runtime/serde/serde-json/TESTING.md**](https://github.com/awslabs/smithy-kotlin/blob/main/runtime/serde/serde-json/TESTING.md)

* [**KotlinDependency.kt**](https://github.com/awslabs/smithy-kotlin/blob/main/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/core/KotlinDependency.kt#L37)

Do these locations need to be updated as well?

So for the runtime build.gradle.kts, same comment. Hopefully this shouldn't be necessary anymore because we default the versions here.

The KotlinDependency.kt shouldn't matter in practice but it's fine to update.

The TESTING.md can be ignored. It's just instructions for how to get started setting up JSONTestSuite. (I don't want to be updating versions more than we need to)

[kggilmer]

For consistency I updated everything except for the json testing, as I think it's better to keep that version sticky unless we want to verify that the tests continue to work from upgrade to upgrade.

The TESTING.md can be ignored. It's just instructions for how to get started setting up JSONTestSuite. (I don't want to be updating versions more than we need to)

agreed