Bump to 0.4.0

There are some version management issues with 0.3.0 so we
need a release to catch up with changes in Boot.

spring-grpc-docs/src/main/antora/modules/ROOT/pages/getting-started.adoc

@@ -199,7 +199,7 @@ If you’re a Maven user, you can use the dependencies by adding the following t
         <dependency>
             <groupId>org.springframework.grpc</groupId>
             <artifactId>spring-grpc-dependencies</artifactId>
-            <version>0.4.0-SNAPSHOT</version>
+            <version>0.4.0</version>
             <type>pom</type>
             <scope>import</scope>
         </dependency>
@@ -214,7 +214,7 @@ As shown in the snippet below this can then be followed by version-less declarat
 [source,gradle]
 ----
 dependencies {
-  implementation platform("org.springframework.grpc:spring-grpc-dependencies:0.4.0-SNAPSHOT")
+  implementation platform("org.springframework.grpc:spring-grpc-dependencies:0.4.0")
 }
 ----