Upgrade to latest docbook gradle plugin

Issue: SPR-10597

build.gradle

@@ -4,7 +4,7 @@ buildscript {
 	}
 	dependencies {
 		classpath("org.springframework.build.gradle:propdeps-plugin:0.0.3")
-		classpath("org.springframework.build.gradle:docbook-reference-plugin:0.2.6")
+		classpath("org.springframework.build.gradle:docbook-reference-plugin:0.2.7")
 	}
 }