version 1.21.2

Changes

• [bolt] #970 Fix #967 Attach AppConfig#getSingleTeamToken() to App#client() - Thanks @seratch
• [slack-api-client] #964 Add preview to chat.unfurl API parameters - Thanks @seratch @cathywise
• [bolt-ktor] #969 Fix #966 Ktor incorrectly decoding event json with unicode symbols - Thanks @tg-dclarke
• [all] Upgrade patch/minor versions of optional dependencies - Thanks @seratch

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• All issues/pull requests: https://github.com/slackapi/java-slack-sdk/milestone/55?closed=1
• All changes: v1.21.1...v1.21.2