spring-boot-gradle-plugin docs use deprecated Gradle API

[halcyon22]

The Spring Boot Gradle Reference Guide uses AbstractArchiveTask.classifier which is deprecated as of Gradle 5.1 and replaced by archiveClassifier. As the plugin requires Gradle 6.8, 6.9, or 7.x, the example should be updated.

https://github.com/spring-projects/spring-boot/blob/main/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-and-jar-classifiers.gradle

bootJar {
    archiveClassifier = 'boot'
}

jar {
    archiveClassifier = ''
}

https://github.com/spring-projects/spring-boot/blob/main/spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-and-jar-classifiers.gradle.kts

tasks.getByName<BootJar>("bootJar") {
    archiveClassifier.set("boot")
}

tasks.getByName<Jar>("jar") {
    archiveClassifier.set("")
}

I can create a PR for this change if preferred.

[snicoll]

@halcyon22 thank you for the report. If you have time for a PR, that would be much appreciated.

[snicoll]

Closing in favor of PR #29611