Handle JavaExec.main deprecation

[ghost]

The property JavaExec.main has been deprecated and is due for removal in Gradle 8.0. This property is used in the PitestTask.

The following stack trace is logged when running on Gradle 7.1.1

The JavaExec.main property has been deprecated. This is scheduled to be removed in Gradle 8.0. Please use the mainClass property instead. See https://docs.gradle.org/7.1.1/dsl/org.gradle.api.tasks.JavaExec.html#org.gradle.api.tasks.JavaExec:main for more details.
        at org.gradle.api.tasks.JavaExec.setMain(JavaExec.java:418)
        at info.solidsoft.gradle.pitest.PitestTask_Decorated.setMain(Unknown Source)
        at org.gradle.api.tasks.JavaExec.setMain(JavaExec.java:112)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at info.solidsoft.gradle.pitest.PitestTask_Decorated.setProperty(Unknown Source)
        at info.solidsoft.gradle.pitest.PitestTask.<init>(PitestTask.groovy:247)
        at info.solidsoft.gradle.pitest.PitestTask_Decorated.<init>(Unknown Source)
        at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
        at org.gradle.internal.instantiation.generator.AsmBackedClassGenerator$InvokeConstructorStrategy.newInstance(AsmBackedClassGenerator.java:2070)
        at org.gradle.internal.instantiation.generator.AbstractClassGenerator$GeneratedClassImpl$GeneratedConstructorImpl.newInstance(AbstractClassGenerator.java:486)
       ... etc

[bondolo]

Proposed a fix in #289 that appears to work correctly.

[szpak]

Thanks @bondolo. I will be a part of "just being released" 1.7.0.