Replace deprecated oss-parent parent POM

Although the parent POM still works without any issues for us, the project is already marked as deprecated.

• https://github.com/sonatype/oss-parents
• https://fedoraproject.org/wiki/Changes/Deprecate_Sonatype_OSS_Parent

To replace the POM, we can remove the following inheritance and add relevant settings in our parent POM XML.

    <parent>
        <groupId>org.sonatype.oss</groupId>
        <artifactId>oss-parent</artifactId>
        <version>9</version>
    </parent>

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 are agreeing to the those rules.

Start by locating the parent POM XML and checking where the org.sonatype.oss:oss-parent:9 inheritance is used across the SDK categories listed in the issue. Remove that inheritance, add the relevant settings to the project parent POM, and verify that the affected modules still build successfully.