Remove legacy Greenplum support from cloudberry-backup

首先，盘点范围中提到的源代码、构建和配置文件、运行时检查、文档、测试以及 CI 工作流。完成的标准是移除 Greenplum 特有的逻辑和兼容性引用，同时继续支持 Cloudberry 行为，并更新受影响的测试和工作流。

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.