Switch from JetBrains annotations to JSpecify.

Change-Id: Ia04b52c36f9779ab15e688e0dd7434644b3a3f7f
diff --git a/jgvariant-tool/pom.xml b/jgvariant-tool/pom.xml
index f4026e4..9d764dc 100644
--- a/jgvariant-tool/pom.xml
+++ b/jgvariant-tool/pom.xml
@@ -59,6 +59,12 @@
       <optional>true</optional>
     </dependency>
     <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+      <scope>provided</scope>
+      <optional>true</optional>
+    </dependency>
+    <dependency>
       <groupId>org.apiguardian</groupId>
       <artifactId>apiguardian-api</artifactId>
       <scope>provided</scope>