Remove outdated compat entries

[gabrieldansereau]

What the pull request does

Following #96, I'm testing the compat entries by running the tests locally. The first one I tried, ArchGDAL, had compat entries for which SimpleSDMLayers didn't work.

Edit: Except ArchGDAL, all compat entries worked. I also added a test for density from StatsPlots, and replaced a color so versions of Plots prior to v1.1.0 can also run the tests.

Type of change

Please indicate the relevant option(s)

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ❇️ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 This change requires a documentation update

Checklist

• The changes are documented
  • The docstrings of the different functions describe the arguments, purpose, and behavior
  • There are examples in the documentation website
• The changes are tested
• The changes do not modify the behavior of the previously existing functions
  • If they do, please explain why and how in the introduction paragraph
• For new contributors - my name and information are added to .zenodo.json
• The Project.toml field version has been updated
  • Change the last number for a v0.0.x series release, a bug fix, or a performance improvement
  • Change the middle number for additional features that do not break the current test suite (unless you fix a bug in the test suite)
  • Change the first number for changes that break the current test suite