fix: debug cleanup

[maxwellt7]

Summary

• Gradle 2.10 → 7.6.4: Old version couldn't parse Java 17 version string
• jcenter() → mavenCentral(): jcenter is sunset/read-only
• Conditional private repo/plugins: Build no longer crashes without SmartThings Artifactory credentials — private deps and deployment plugins activate only when smartThingsArtifactoryUserName is set in gradle.properties
• Compile → Implementation: Migrated deprecated dependency configurations
• Fixed configure/unconfigure tasks: Original Exec tasks had multiple commandLine calls but only the last one executed; now uses doLast with exec blocks so all commands run
• Dynamic class loading for Slack classes: Avoids compile-time resolution of FileUpload/Message when private libs aren't on classpath
• Added gradle.properties: Placeholder for credentials
• Added .env.example: Documents all required configuration

Test plan

• ./gradlew build succeeds without credentials (clean build)
• ./gradlew tasks lists all expected tasks including devicetypesClasses and smartappsClasses
• With valid SmartThings Artifactory credentials, private deps resolve and deployment plugins load

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