Move the application convention properties into the application { } block

ApplicationPluginConvention is deprecated since Gradle 8.2 and removed in Gradle 9, so assigning applicationName, applicationDefaultJvmArgs or executableDir on the project warns on Gradle 8.14 in Groovy, fails on Gradle 9 in Groovy with Could not set unknown property, and does not compile in Kotlin on Gradle 8.14 or 9. The assignment moves into the application { } block. Kotlin uses configure<JavaApplication> { } where the script's own plugins { } block does not apply application, because only then does the type-safe accessor exist. +=, reads, and a script's own def, val or ext of the same name are left alone. See the Gradle upgrade guide for more information.