Remove legacy Greenplum support from cloudberry-backup

## Background

The `cloudberry-backup` project was originally derived from Greenplum backup tooling and still contains legacy code paths, configurations, and compatibility logic intended to support Greenplum Database environments.

Since Apache Cloudberry has evolved as an independent project with its own architecture, release cycle, and compatibility guarantees, continuing to maintain legacy Greenplum support introduces unnecessary complexity and maintenance burden.

## Motivation

Removing legacy Greenplum support will provide several benefits:

- Simplify the codebase and reduce maintenance overhead
- Improve code clarity and long-term maintainability
- Align the project strictly with Apache Cloudberry’s architecture and roadmap
- Reduce testing and validation scope
- Avoid confusion about supported platforms

The `cloudberry-backup` project should be focused solely on serving Apache Cloudberry users.

## Scope of Changes

Expected areas of modification include:

- Source code cleanup and removal of Greenplum-specific logic
- Update build, configuration, and runtime checks
- Documentation updates
- Test suite cleanup
- CI workflow adjustments if applicable

## Compatibility Impact

This change will be backward-incompatible for Greenplum users. However, since this repository is now part of the Apache Cloudberry ecosystem, focusing exclusively on Cloudberry is consistent with project goals.

まず、対象範囲で言及されているソースコード、ビルドおよび設定ファイル、ランタイムチェック、ドキュメント、テスト、CIワークフローを洗い出します。Greenplum固有のロジックと互換性に関する参照が削除され、Cloudberryの動作が引き続きサポートされ、影響を受けるテストとワークフローが更新されれば完了です。

索引モデルが issue の本文から書いたものです。