Fix build-tools/reaper source/target compatibility to be JDK-11 (#2596) (#2605)

Signed-off-by: Andriy Redko <andriy.redko@aiven.io>
(cherry picked from commit d4ce87b)

Co-authored-by: Andriy Redko <andriy.redko@aiven.io>

Author: opensearch-trigger-bot[bot]

buildSrc/reaper/build.gradle

@@ -11,6 +11,9 @@
 
 apply plugin: 'java'
 
+targetCompatibility = JavaVersion.VERSION_11
+sourceCompatibility = JavaVersion.VERSION_11
+
 jar {
   archiveFileName = "${project.name}.jar"
   manifest {