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.

Start by inventorying the source code, build and configuration files, runtime checks, documentation, tests, and CI workflows mentioned in the scope. Done means Greenplum-specific logic and compatibility references are removed, Cloudberry behavior remains supported, and the affected tests and workflows are updated.

Written by the indexing model from the issue text.