Support removal of main property in Gradle 8.0

[tresat]

• The main property is going to be replaced by mainClass in the JavaExec class.
• Dynamically attempt to continue calling the original property, but upon failure call the new property instead.

This addresses https://github.com/asciidoctor/asciidoctor-gradle-plugin/issues/602 and (somewhat) https://github.com/asciidoctor/asciidoctor-gradle-plugin/issues/611.