Switch from JetBrains annotations to JSpecify.

Change-Id: Ia04b52c36f9779ab15e688e0dd7434644b3a3f7f
diff --git a/jgvariant-core/pom.xml b/jgvariant-core/pom.xml
index 89ccb6f..e8730a3 100644
--- a/jgvariant-core/pom.xml
+++ b/jgvariant-core/pom.xml
@@ -45,6 +45,11 @@
       <optional>true</optional>
     </dependency>
     <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+      <optional>true</optional>
+    </dependency>
+    <dependency>
       <groupId>org.apiguardian</groupId>
       <artifactId>apiguardian-api</artifactId>
       <optional>true</optional>