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

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.

Comment From: snicoll

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

Comment From: snicoll

Closing in favor of PR #29611