Remove deprecated calls, bump dependencies for OTP 21+

[SamJakob]

• Updates the worker call to the new supervisor tree API
• Swaps calls to Logger.warn for Logger.warning as the former is deprecated.
• Replace Mix config options with new Config module (Elixir 1.11+ because of use of the recommended config_env/0) function.
• CI tests with GitHub Actions passing for latest Elixir/OTP combo as well as [Elixir 1.11, OTP 21] and [Elixir 1.11, OTP 24].
• Fixes a documentation typo that made the last function call (change_task_after/3)'s documentation a bit scuffed