gradle: desugar retrofuture and remove old deps

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tunnel/src/main/java/com/wireguard/android/backend/WgQuickBackend.java

@@ -32,8 +32,6 @@ import java.util.Objects;
 import java.util.Set;
 
 import androidx.annotation.Nullable;
-import java9.util.stream.Collectors;
-import java9.util.stream.Stream;
 
 /**
  * WireGuard backend that uses {@code wg-quick} to implement tunnel configuration.
@@ -67,7 +65,7 @@ public final class WgQuickBackend implements Backend {
             return Collections.emptySet();
         }
         // wg puts all interface names on the same line. Split them into separate elements.
-        return Stream.of(output.get(0).split(" ")).collect(Collectors.toUnmodifiableSet());
+        return Set.of(output.get(0).split(" "));
     }
 
     @Override