Rename plugin PDF reference doc files

This commit renames the PDF documentation files generated for the
build plugins when the distribution archive is created.

See gh-20829

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/build.gradle

@@ -99,6 +99,7 @@ task zip(type: Zip) {
 	duplicatesStrategy "fail"
 	from(asciidoctorPdf.outputDir) {
 		into "reference/pdf"
+		rename "index.pdf", "${project.name}-reference.pdf"
 	}
 	from(asciidoctor.outputDir) {
 		into "reference/html"

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/build.gradle

@@ -99,6 +99,7 @@ task zip(type: Zip) {
 	duplicatesStrategy "fail"
 	from(asciidoctorPdf.outputDir) {
 		into "reference/pdf"
+		rename "index.pdf", "${project.name}-reference.pdf"
 	}
 	from(asciidoctor.outputDir) {
 		into "reference/html"