Expose [JSpecify annotations](jspecify/jspecify#239) in our GWT artifact.

This requires an update to [GWT 2.12.1](https://www.gwtproject.org/release-notes.html#Release_Notes_2_12_1), as discussed in the release notes.

RELNOTES=Made our usages of nullness annotations available in our GWT artifact. GWT users will need to upgrade to [GWT 2.12.1](https://www.gwtproject.org/release-notes.html#Release_Notes_2_12_1), which makes GWT as tolerant of Java 8 type-use annotations as it is of other annotations.
PiperOrigin-RevId: 711487270

Author: cpovirk

guava-gwt/pom.xml

@@ -17,7 +17,7 @@
     This project includes GWT-friendly sources.
   </description>
   <properties>
-    <gwt.version>2.11.0</gwt.version>
+    <gwt.version>2.12.1</gwt.version>
     <gwt.plugin.version>2.10.0</gwt.plugin.version>
     <gwt.logLevel>WARN</gwt.logLevel>
   </properties>
@@ -333,24 +333,6 @@
                     <contains text="@GwtCompatible"/>
                   </fileset>
                 </copy>
-                <replace token="@NullMarked" value="">
-                  <fileset dir="${project.build.directory}">
-                    <include name="guava-gwt-sources/**/*.java"/>
-                    <include name="guava-test-gwt-sources/**/*.java"/>
-                  </fileset>
-                </replace>
-                <replace token="@Nullable" value="">
-                  <fileset dir="${project.build.directory}">
-                    <include name="guava-gwt-sources/**/*.java"/>
-                    <include name="guava-test-gwt-sources/**/*.java"/>
-                  </fileset>
-                </replace>
-                <replace token="@NonNull" value="">
-                  <fileset dir="${project.build.directory}">
-                    <include name="guava-gwt-sources/**/*.java"/>
-                    <include name="guava-test-gwt-sources/**/*.java"/>
-                  </fileset>
-                </replace>
               </target>
             </configuration>
           </execution>