Strip @NullMarked from GWT/J2CL artifact.

We already strip `@Nullable` because of [a GWT bug with type-use annotations](gwtproject/gwt#10020) (recently fixed!). But because of our usages of `@NullMarked`, the `@Nullable` stripping makes all types non-null. To avoid that, we strip `@NullMarked`, too.

RELNOTES=Changed our GWT/J2CL artifact to omit usages of `@NullMarked`. This was making all our types non-null in those environments, since we don't yet use `@Nullable` in the GWT/J2CL artifact.
PiperOrigin-RevId: 700266827

core/pom.xml

@@ -215,6 +215,11 @@
                     <include name="**/super/**/*.java"/>
                   </fileset>
                 </replace>
+                <replace token="@NullMarked" value="">
+                  <fileset dir="${project.build.directory}/gwt-sources">
+                    <include name="**/super/**/*.java"/>
+                  </fileset>
+                </replace>
               </target>
             </configuration>
           </execution>