Install or Defer 3.0.2

Changed

• removed CatalogURL check from macOS 11+ (custom catalog URL definition is deprecated in Catalina and not supported in Big Sur)
• removed all Python calls to prepare for eventual Python runtime removal in future macOS releases
  • replaced managed preferences read commands with defaults read pointed at the /Library/Managed Preferences/ file path
• restored StartInterval attribute read
  • added comment for context of purpose
• replaced [ERROR] with ❌ ERROR: in script output
• added full binary paths (except for built-ins)
• double-quote-surrounded file paths and variables
• removed error code parsing to avoid ShellCheck flags
• removed shellcheck disable definitions
• updated ShellCheck job to use latest version of azohra/shell-linter
• separated optional content from required in README for improved legibility