Use Java 11 source code

https://developer.android.com/studio/releases/gradle-plugin#java-11

matrix-sdk-android/build.gradle

@@ -72,12 +72,12 @@ android {
     }
 
     compileOptions {
-        sourceCompatibility JavaVersion.VERSION_1_8
-        targetCompatibility JavaVersion.VERSION_1_8
+        sourceCompatibility JavaVersion.VERSION_11
+        targetCompatibility JavaVersion.VERSION_11
     }
 
     kotlinOptions {
-        jvmTarget = "1.8"
+        jvmTarget = "11"
     }
 
     sourceSets {