Update the project version to 1.38.1-SNAPSHOT

Author: armerian

gradle.properties

@@ -1,5 +1,5 @@
 group=com.linecorp.armeria
-version=1.37.1-SNAPSHOT
+version=1.38.1-SNAPSHOT
 projectName=Armeria
 projectUrl=https://armeria.dev/
 projectDescription=Asynchronous HTTP/2 RPC/REST client/server library built on top of Java 8, Netty, Thrift and gRPC