Migrate to Backstage new frontend system

[talissoncosta]

Summary

Migrate the Flagsmith plugin to use Backstage's new frontend system as the default export, replacing the legacy createPlugin API.

Changes

• Remove legacy frontend system files (plugin.ts, routes.ts)
• Use createFrontendPlugin from @backstage/frontend-plugin-api as the default export
• Implement EntityContentBlueprint for the flags tab
• Implement EntityCardBlueprint for overview and usage cards
• Remove separate /alpha export path (new system is now the default)

Breaking Changes

• Plugin now requires Backstage apps using the new frontend system
• Legacy flagsmithPlugin named export removed
• Direct component exports (FlagsTab, FlagsmithOverviewCard, FlagsmithUsageCard) removed from package root

Dependencies

• Depends on Migrate Backstage plugin to proxy architecture with lazy loading #6420 (Proxy architecture)

Parent Epic

• Epic: Implement Backstage plugin #5641

[talissoncosta]

PR: Flagsmith/flagsmith-backstage-plugin#3