Gradle: Demand Java 15.

Change-Id: I1fc64ba07bbd93ed139142a484d65b66ebc4a704
diff --git a/blog/build.gradle.kts b/blog/build.gradle.kts
index 6a57a3f..592bdff 100644
--- a/blog/build.gradle.kts
+++ b/blog/build.gradle.kts
@@ -29,8 +29,8 @@
 version = "1.0.0-SNAPSHOT"
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_11
-    targetCompatibility = JavaVersion.VERSION_11
+    sourceCompatibility = JavaVersion.VERSION_15
+    targetCompatibility = JavaVersion.VERSION_15
 }
 
 tasks.withType<JavaCompile> {