Fix incorrect usage of gp_versionstr in pg_upgrade version check

[yjhjstz]

Previously, the Cloudberry-specific version string was incorrectly written into versionstr instead of gp_versionstr, breaking the fallback comparison logic in check_exec(). This caused version checks to fail when the program output matched the expected Apache Cloudberry format.

Fixes #ISSUE_Number

What does this PR do?

Type of Change

• [ ] Bug fix (non-breaking change)
• [ ] New feature (non-breaking change)
• [ ] Breaking change (fix or feature with breaking changes)
• [ ] Documentation update

Breaking Changes

Test Plan

• [ ] Unit tests added/updated
• [ ] Integration tests added/updated
• [ ] Passed make installcheck
• [ ] Passed make -C src/test installcheck-cbdb-parallel

Impact

Performance:

User-facing changes:

Dependencies:

Checklist

• [ ] Followed contribution guide
• [ ] Added/updated documentation
• [ ] Reviewed code for security implications
• [ ] Requested review from cloudberry committers

Additional Context

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