Fix instructions for running conformance tests

- The server-servlet app depends on mcp, which must be either installed
  or included in the compile artifact for `mvn exec:java` to pick it up.
- Change the build instructions to build from root, use a `-pl` flag to
  target the servlet app.
- Disable the exec plugin from the root pom.

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

pom.xml

@@ -263,6 +263,15 @@
 				<artifactId>maven-deploy-plugin</artifactId>
 				<version>${maven-deploy-plugin.version}</version>
 			</plugin>
+			<plugin>
+				<groupId>org.codehaus.mojo</groupId>
+				<artifactId>exec-maven-plugin</artifactId>
+				<version>3.1.0</version>
+				<configuration>
+					<skip>true</skip>
+					<mainClass>ignored</mainClass>
+				</configuration>
+			</plugin>
 		</plugins>
 	</build>