Migrate to central-publishing-maven-plugin.

This should deal with the new Sonatype part of #401.

It is based closely on cl/781199945.

I will also update the CI repository secrets at https://github.com/google/jimfs/settings/secrets/actions as described in https://central.sonatype.org/publish/generate-portal-token/. I expect to need that for releases (since we still produce them on our local machines), so I put the text it generated into my `.m2/settings.xml`. However, I notice that it produces the literal output `<id>${server}</id>`, which seems iffy. I am trying changing it to `central`, and we'll see what happens when I try to publish a release. (I will eventually have to do something similar for my `.gradle/gradle.properties` for _JSpecify_ releases.)

RELNOTES=n/a
PiperOrigin-RevId: 782079419

Author: cpovirk

pom.xml

@@ -82,14 +82,9 @@
   <distributionManagement>
     <snapshotRepository>
       <id>sonatype-nexus-snapshots</id>
-      <name>Sonatype Nexus Snapshots</name>
-      <url>https://oss.sonatype.org/content/repositories/snapshots/</url>
+      <url>https://central.sonatype.com/repository/maven-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>
 
   <properties>
@@ -380,6 +375,15 @@
               </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>
+            </configuration>
+          </plugin>
         </plugins>
       </build>
     </profile>