[AC-53] Confluence 8.9 Compatibility + Java 11 minVersion + Platform BOM 6.5 + REST V2

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Story

Make App compatible to Confluence 8.9 and handle Atlassian Code deprecations.

Acceptance Criteria

• ✅ Continued cleanup and preparations for Confluence 9 as started with [AC-51] Confluence 8.8 Compatibility + Java 11 minVersion + Platform BOM 6.5 #51
• ✅ Java 11 min version
• ✅ Confluence 8.9 min version
• ✅ REST V2 migration
• ✅ Platform 6.5.9
• ✅ Removed Spring Scanner and migrated to Spring-Java-Config
• ✅ Replaced InitializationBean with @Bean(initMethod = "init")

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Released to Atlassian Marketplace with v5.1.0