build(maven): move extension declared in module at root level

Having multiple submodules declaring extensions cause the
maven-deploy-plugin to not deploy artifacts when using -DdeployAtEnd=true.
See apache/maven-deploy-plugin#1

Author: Jujube Orange

.mvn/extensions.xml

@@ -1,5 +1,10 @@
 <extensions xmlns="http://maven.apache.org/EXTENSIONS/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.0.0 http://maven.apache.org/xsd/core-extensions-1.0.0.xsd">
+    <extension>
+        <groupId>kr.motd.maven</groupId>
+        <artifactId>os-maven-plugin</artifactId>
+        <version>1.5.0.Final</version>
+    </extension>
     <extension>
         <groupId>me.qoomon</groupId>
         <artifactId>maven-git-versioning-extension</artifactId>

robert-crypto-grpc-server-messaging/pom.xml

@@ -69,13 +69,6 @@
 	</dependencies>
 
 	<build>
-		<extensions>
-			<extension>
-				<groupId>kr.motd.maven</groupId>
-				<artifactId>os-maven-plugin</artifactId>
-				<version>1.5.0.Final</version>
-			</extension>
-		</extensions>
 		<plugins>
 			<plugin>
 				<groupId>org.xolstice.maven.plugins</groupId>

robert-crypto-grpc-server/pom.xml

@@ -154,13 +154,6 @@
     </dependencies>
 
 	<build>
-		<extensions>
-			<extension>
-				<groupId>kr.motd.maven</groupId>
-				<artifactId>os-maven-plugin</artifactId>
-				<version>1.5.0.Final</version>
-			</extension>
-		</extensions>
 		<plugins>
 			<plugin>
 				<groupId>org.xolstice.maven.plugins</groupId>