Bump japicmp plugin to 0.2.6 and activate source level compat check

build.gradle

@@ -34,7 +34,7 @@ plugins {
   id 'org.asciidoctor.convert' version '1.5.6'
   id "me.champeau.gradle.jmh" version "0.4.4" apply false
   id "org.jetbrains.dokka" version "0.9.15"
-  id "me.champeau.gradle.japicmp" version "0.2.5"
+  id "me.champeau.gradle.japicmp" version "0.2.6"
 }
 
 apply from: "gradle/doc.gradle"
@@ -270,6 +270,7 @@ project('reactor-core') {
 	newClasspath = files(jar.archivePath)
 	onlyBinaryIncompatibleModified = true
 	failOnModification = true
+	failOnSourceIncompatibility = true
 	txtOutputFile = file("${project.buildDir}/reports/japi.txt")
 	ignoreMissingClasses = true
 	includeSynthetic = true
@@ -441,6 +442,7 @@ project('reactor-test') {
 	newClasspath = files(jar.archivePath)
 	onlyBinaryIncompatibleModified = true
 	failOnModification = true
+	failOnSourceIncompatibility = true
 	txtOutputFile = file("${project.buildDir}/reports/japi.txt")
 	ignoreMissingClasses = true
 	includeSynthetic = true