Downgrade H2 to 1.4.192 for Java 6 compatibility

Closes #107 (closed)

mailbox-core/build.gradle

@@ -28,7 +28,8 @@ dependencies {
     implementation "io.ktor:ktor-jackson:$ktor_version"
     api "org.slf4j:slf4j-api:1.7.32"
 
-    implementation 'com.h2database:h2:1.4.200'
+    //noinspection GradleDependency
+    implementation 'com.h2database:h2:1.4.192' // The last version that supports Java 1.6
 
     // Base 32
     implementation 'dev.keiji.rfc4648:rfc4648:1.0.0'