Update j2objc-annotations to use Sonatype Central Publishing.

This change upgrades the Maven compiler source/target to Java 11, updates the project version, and replaces the manual Sonatype Nexus staging/release process with the `central-publishing-maven-plugin` for automated publishing to Maven Central. The opensource documentation has been updated to reflect the new publishing workflow.

PiperOrigin-RevId: 795130224

Author: tomball

annotations/pom.xml

@@ -19,14 +19,14 @@
   <modelVersion>4.0.0</modelVersion>
 
   <properties>
-    <maven.compiler.source>1.7</maven.compiler.source>
-    <maven.compiler.target>1.7</maven.compiler.target>
+    <maven.compiler.source>11</maven.compiler.source>
+    <maven.compiler.target>11</maven.compiler.target>
   </properties>
 
   <groupId>com.google.j2objc</groupId>
   <artifactId>j2objc-annotations</artifactId>
   <packaging>jar</packaging>
-  <version>3.0.0-SNAPSHOT</version>
+  <version>3.1-SNAPSHOT</version>
 
   <name>J2ObjC Annotations</name>
   <description>
@@ -59,19 +59,6 @@
     </developer>
   </developers>
 
-  <distributionManagement>
-    <snapshotRepository>
-      <id>sonatype-nexus-snapshots</id>
-      <name>Sonatype Nexus Snapshots</name>
-      <url>https://oss.sonatype.org/content/repositories/snapshots/</url>
-    </snapshotRepository>
-    <repository>
-      <id>sonatype-nexus-staging</id>
-      <name>Nexus Release Repository</name>
-      <url>https://oss.sonatype.org/service/local/staging/deploy/maven2/</url>
-    </repository>
-  </distributionManagement>
-
   <build>
     <plugins>
       <plugin>
@@ -153,6 +140,16 @@
           </execution>
         </executions>
       </plugin>
+      <plugin>
+        <groupId>org.sonatype.central</groupId>
+        <artifactId>central-publishing-maven-plugin</artifactId>
+        <version>0.8.0</version>
+        <extensions>true</extensions>
+        <configuration>
+           <publishingServerId>central</publishingServerId>
+           <autoPublish>true</autoPublish>
+        </configuration>
+      </plugin>
     </plugins>
   </build>
 </project>