Enable configuration of Checker Framework version

The plugin currently seems hard-coded to use version 2.4.0 of the Checker Framework. Many bugs have been fixed, and features added, since then.

Please provide a configuration setting that I can use in my `build.gradle` file to specify what version of the Checker Framework the plugin will use. (It would also be nice to change the default version to 2.6.0.)

Start with the plugin's Gradle configuration and locate the hard-coded Checker Framework 2.4.0 reference, then inspect how settings from a consumer build.gradle are exposed. Done means a consumer can select the framework version and the requested default-version change is addressed; verify the result with the project's available checks.

Written by the indexing model from the issue text.