Replace deprecated oss-parent parent POM

[seratch]

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. :bow:

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