README: Show Gradle 8.7 in headline

This has the last version that passed CI testing,
but expecting the current latest 8.9 to pass with
dokka / asciidoctor upgrades of recent commits.

README.adoc

@@ -22,7 +22,7 @@ image:{img-build-status}[CI-Gradle-build,link={uri-build-status}]
 
 #Plugin artifact supports `Java 8`, `Java 11`, `Java 17` and `Java 21`#
 
-#For use with `Gradle 5.6` through `Gradle 8.5`#
+#For use with `Gradle 5.6` through `Gradle 8.7`#
 endif::[]
 
 = Gradle XJC Plugin