chore: set Java source and target compatibility to version 1.8

Author: Fluf22

clients/algoliasearch-client-java/algoliasearch/build.gradle

@@ -8,6 +8,11 @@ repositories {
     mavenCentral()
 }
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+}
+
 dependencies {
     implementation 'com.google.code.findbugs:jsr305:3.0.2'
     api 'com.squareup.okhttp3:okhttp:5.1.0'