Bump Kotlin and Dokka versions to latest release

[kggilmer]

Companion PRs:

• Bump Kotlin version to latest release aws-crt-kotlin#37
• Bump Kotlin and Dokka versions to latest release smithy-lang/smithy-kotlin#506

Issue #

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:

• build.gradle.kts
• examples/build.gradle.kts

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:

* [**build.gradle.kts**](https://github.com/awslabs/aws-sdk-kotlin/blob/main/build.gradle.kts#L8)

* [**examples/build.gradle.kts**](https://github.com/awslabs/aws-sdk-kotlin/blob/main/examples/build.gradle.kts#L2)

Do these locations need to be updated as well?

Good catch. I think (hope) this is irrelevant now since we set the default plugin versions here