Migrate to use System.Logger rather than SLF4J-API (better support for jlink / module-path)

[Rob Bygrave]

This migrates to use the Java System.Logger API rather than slf4j-api.

This reduces the dependencies for this library and more specifically makes it easier for people wanting to use jlink and module-path as slf4j-api 1.7 only has automatic module name and not a module-info (and slf4j-api 2.x is still alpha).

This change messages logged to System.Logger will by default go to JUL unless there is something registered.

If you already have a JUL bridge then you don't need to do anything and log messages will already go via that path.

Otherwise, we can use:

    <dependency>
      <groupId>io.avaje</groupId>
      <artifactId>avaje-slf4j-jpl</artifactId>
      <version>1.0</version>
    </dependency>

... to have the log messages sent to slf4j-api and then onto whatever logging backend is setup.