Bump to GMT 6.7.0

[seisman]

🎉 GMT 6.7.0 has been released! It is installable from the conda-forge channel using the following command:

conda install -c conda-forge gmt=6.7.0

To-Do for bumping the GMT version in CI:

• Bump the GMT version in CI (1 PR)
  • Update environment.yml
  • Update ci/requirements/docs.yml
  • Update .github/workflows/benchmarks.yml
  • Update .github/workflows/cache_data.yaml
  • Update .github/workflows/ci_doctests.yaml
  • Update .github/workflows/ci_docs.yml
  • Update .github/workflows/ci_tests.yaml
  • Add the legacy GMT version to .github/workflows/ci_tests_legacy.yaml
  • Update GMT 6.x minor version for extlinks urls in doc/conf.py
  • Update the GMT-Ghostscript compatibility note in doc/install.md
• Fix failing tests (1 or more PRs)
• Fix failing doctests reported in the "Doctest" workflow
• Run grep -r "# TODO(GMT.*)" --include="*.py" . to find TODO items related to GMT
  • Remove related workarounds/patches
  • Fix tests with pytest xfail/skipif markers that now pass
• Update URLs to manually inserted images of the GMT documentation
• Update this checklist if necessary

To-Do for bumping the minimum required GMT version:

• Bump the minimum required GMT version (1 PR)
  • Update required_gmt_version in pygmt/clib/__init__.py
  • Remove unsupported GMT version from .github/workflows/ci_tests_legacy.yaml
  • Ensure minimum required version is correct in doc/minversions.md
• Run grep -r "# TODO(GMT.*)" --include="*.py" . to find TODO items related to GMT
  • Remove related workarounds/patches
  • Fix tests with pytest xfail/skipif markers that now pass
• Update deprecated syntax in source code and examples based on the GMT Changelog
• Update this checklist if necessary