Upgrade slf4j-api to 2.0

[seratch]

slf4j project released a new major version - 2.0.

According to the migration guide, it's safe to upgrade the dependency in this project but it requires a minor version bump of this SDK. EDIT: I might be wrong. Perhaps, if we release a new version with slf4j-api 2.0 dependency, real-world apps such as Spring Boot web apps would be suffered from behavior changes. We should hold off upgrading to 2.0 for the time being.

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please make sure if this topic is specific to this SDK. For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

[seratch]

Still waiting for logback-classic 1.3.0 release: https://github.com/qos-ch/logback/tags

[seratch]

EDIT: I might be wrong. Perhaps, if we release a new version with slf4j-api 2.0 dependency, real-world apps such as Spring Boot web apps would be suffered from behavior changes. We should hold off upgrading to 2.0 for the time being.

[seratch]

Another factor we should consider for this upgrade is that logback-classic 1.4 and perhaps other implementations of sfl4j-api v2 no longer supports Java 1.8. Yes, the version is long-lived old version but this SDK still supports the version for broader audience.