Remove sealed from ManagedCache abstract class. (#93)

It prevents us to use it

Author: guizmaii

zio-cache/shared/src/main/scala/zio/cache/ManagedCache.scala

@@ -9,7 +9,7 @@ import java.util
 import java.util.concurrent.atomic.{AtomicBoolean, AtomicInteger, LongAdder}
 import scala.jdk.CollectionConverters._
 
-sealed abstract class ManagedCache[-Key, +Error, +Value] {
+abstract class ManagedCache[-Key, +Error, +Value] {
 
   /**
    * Returns statistics for this cache.