Fix: attribute "gradle-plugin" of android config is never read

[giuppe]

Adding the tag:

<android gradle-plugin="8.7.0" />

to Project.xml has no effect right now, while in AndroidPlatform.hx it is considered valid:

lime/tools/platforms/AndroidPlatform.hx

Line 477 in 36b14e9

context.ANDROID_GRADLE_PLUGIN = project.config.getString("android.gradle-plugin", "7.3.1");

This is because the ProjectXMLParser is missing the related case - there is one for gradle-version but nothing for gradle-plugin.