solution: upgrade Kotlin to 1.5

src/main/kotlin/io/emeraldpay/dshackle/upstream/bitcoin/BitcoinMultistream.kt

@@ -27,6 +27,7 @@ import org.slf4j.LoggerFactory
 import org.springframework.context.Lifecycle
 import reactor.core.publisher.Mono
 
+@Suppress("UNCHECKED_CAST")
 open class BitcoinMultistream(
         chain: Chain,
         val upstreams: MutableList<BitcoinUpstream>,
@@ -108,6 +109,7 @@ open class BitcoinMultistream(
         return upstreams.flatMap { it.getLabels() }
     }
 
+    @Suppress("UNCHECKED_CAST")
     override fun <T : Upstream> cast(selfType: Class<T>): T {
         if (!selfType.isAssignableFrom(this.javaClass)) {
             throw ClassCastException("Cannot cast ${this.javaClass} to $selfType")