ASF: Update Apache compliance and branding migration

[tuhaihe]

This commit implements comprehensive changes to align the project with Apache Software Foundation incubation requirements and complete the migration from Greenplum to Apache Cloudberry branding.

• Add DISCLAIMER file as required for Apache incubating projects

• Update LICENSE file with comprehensive list of 245 files containing Apache License headers, organized by module (FDW, External Table, Server, Documentation, CI/Test templates)

• Add Apache License headers to GitHub workflow files

• Update CONTRIBUTING.md with Apache project contribution guidelines

• Update README.md with Apache Cloudberry branding and simplified content

• Update documentation templates in docs/content/ to use Cloudberry

• Update package specifications and installation scripts:

  • Rename package from pxf-gpX to cloudberry-pxf
  • Update installation paths to /usr/local/cloudberry-pxf-[VERSION]
  • Remove legacy pxf-gp7.spec and pxf-cbdb1.spec files
  • Update DEBIAN package control files with new naming

• Update automation and testing documentation

• Migrate server scripts and Java components references

• Standardize package configuration based on cloudberry-pxf.spec

• Update CI/CD workflows with proper Apache licensing

• Clean up legacy CI documentation (remove ci/README.md)

• Update build system references in Makefile

• Enhance installation scripts to support both Cloudberry 2.0 and 2.1+

• Add transition guide for Cloudberry migration

• Update all user-facing documentation with correct branding

• Simplify README.md focusing on essential information

• Update book configuration for documentation generation

This change ensures full compliance with Apache incubation requirements while completing the transition to Apache Cloudberry ecosystem.

closes: #ISSUE_Number

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[MisterRaindrop]

LGTM

[leborchuk]

Reviewed, most changes added LICENSE info to align ASF policy.

Debian and RPM packaging could be further improved, but this should be discussed elsewhere. I also agree with removing dependencies from the cloudberry-db package as pxf does not depend on the main cloudberry package.

[tuhaihe]

Reviewed, most changes added LICENSE info to align ASF policy.

Debian and RPM packaging could be further improved, but this should be discussed elsewhere. I also agree with removing dependencies from the cloudberry-db package as pxf does not depend on the main cloudberry package.

Thanks @leborchuk for your review. Now I tested a few times, and the RPM/DEB packages can be generated and installed successfully. Let's merge this PR first. If any more questions, we can fix them later.