Bump to GMT 6.4.0

[seisman]

🎉 GMT 6.4.0 has been released! It is installable from the
conda-forge channel using the following command:

conda install -c conda-forge gmt=6.4.0

To-Do for bumping the GMT version in CI:

• Bump the GMT version in CI (1 PR) (Bump the GMT version in CI to 6.4.0 #1990)
  • Update .github/workflows/cache_data.yaml
  • Update .github/workflows/ci_docs.yml
  • Update .github/workflows/ci_tests.yaml
  • Update ci/requirements/docs.yml
  • Update environment.yml
• Fix failing tests (1 or more PRs) (load_earth_relief() now returns gridline-registrated grids by default in GMT 6.4 #2052)
• Fix xfail pytest markers on tests that are now xpass