Remove legacy CI and unused files from diskquota

Cleanup the invalid files after importing diskquota extension from upstream.

The specific changes include:
1. Remove .github directory: The upstream GitHub Actions workflows are
   no longer applicable in the Cloudberry repository.
2. Remove .gitmessage, .editorconfig, .clang-format: Code style and
   commit templates should follow Cloudberry's main repository standards.
3. Remove SECURITY.md: Legacy security policy.
4. Remove concourse directory: Legacy CI scripts.

This cleanup makes the extension structure cleaner and more consistent
with other contrib modules.

Author: tuhaihe