Refactor for Chef 14 and beyond

[jeffbyrnes]

• Update to ZooKeeper v3.6.1 (closes #223, closes #199)
• Drop Chef 13 support b/c it is now 2 years EOL
  • Pin to java < 7.0.0 to maintain Chef 14 compatibility
• Add testing on Ubuntu 20.04 & CentOS 8
• Drop recipes, per long-standing deprecation notice, and reduce cookbook to all resources
• Rewrite unit tests for very basic coverage
• Switch to GitHub Actions for test automation
• Drop support for runit & upstart
  • This was originally done b/c supporting a multitude of init systems was too great a burden
  • SystemD is now the standard across all the distros we support, so we can decrease the maintenance overhead of this cookbok
• Use the systemd_unit resource available in Chef >= 12.11