Replace use of classifier with archiveClassifier

classifier has been deprecated so the docs should be using its
replacement, archiveClassifier, instead.

See gh-29611

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-and-jar-classifiers.gradle

@@ -5,11 +5,11 @@ plugins {
 
 // tag::classifiers[]
 bootJar {
-	classifier = 'boot'
+	archiveClassifier = 'boot'
 }
 
 jar {
-	classifier = ''
+	archiveClassifier = ''
 }
 // end::classifiers[]
 

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-and-jar-classifiers.gradle.kts

@@ -7,11 +7,11 @@ plugins {
 
 // tag::classifiers[]
 tasks.getByName<BootJar>("bootJar") {
-	classifier = "boot"
+	archiveClassifier.set("boot")
 }
 
 tasks.getByName<Jar>("jar") {
-	classifier = ""
+	archiveClassifier.set("")
 }
 // end::classifiers[]