Drop GMT 6.4.0 support and add GMT 6.6.0 support

[seisman]

GMT 6.6.0 was released a few hours ago, and GMT 6.4 was released in June, 2022.

Following our support policy,

Drop support for GMT versions 3 years after their initial release, while ensuring at least two latest minor versions remain supported.

I think it's time to drop the GMT 6.4.0 support.

• GMT Legacy Tests: Bump from 6.4.0 to 6.5.0 Drop GMT 6.4 support  #4127
• GMT Tests: Bump from 6.5.0 to 6.6.0 Bump the GMT version to 6.6.0 in CI #4018
• Remove tests/codes related to GMT 6.4.0 Remove deprecated codes after dropping GMT 6.4 support #4128
• Bump minimum support GMT version to 6.5.0 Drop GMT 6.4 support  #4127
• Update the issue template bump_gmt_checklist.md Update issue template for bumping GMT versions #4019
• Unpin ghostscript in ci_tests_legacy.yaml Drop GMT 6.4 support  #4127

[seisman]

To-Do for bumping the GMT version in CI:

• Bump the GMT version in CI (1 PR) Bump the GMT version to 6.6.0 in CI #4018
  • 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 GMT 6.x minor version in .github/workflows/check-links.yml
• Fix failing tests (1 or more PRs)
• Fix failing doctests reported in the "Doctest" workflow
• Fix xfail pytest markers on tests that are now xpass

To-Do for bumping the minimum required GMT version:

• Bump the minimum required GMT version (1 PR) Drop GMT 6.4 support  #4127
  • Update required_gmt_version in pygmt/clib/__init__.py
  • Update test_get_default in pygmt/tests/test_clib.py
  • Update minimum required versions in doc/minversions.md
  • Remove unsupported GMT version from .github/workflows/ci_tests_legacy.yaml
• Remove xfail pytest markers on tests that are now xpass
• Update deprecated syntax in source code and examples based on the GMT Changelog