Switch from JetBrains annotations to JSpecify.

Change-Id: Ia04b52c36f9779ab15e688e0dd7434644b3a3f7f
diff --git a/jgvariant-parent/pom.xml b/jgvariant-parent/pom.xml
index 1e41527..815dff2 100644
--- a/jgvariant-parent/pom.xml
+++ b/jgvariant-parent/pom.xml
@@ -76,6 +76,7 @@
     <inject-resources.version>0.3.3</inject-resources.version>
     <jetbrains-annotations.version>24.1.0</jetbrains-annotations.version>
     <jqwik.version>1.9.0</jqwik.version>
+    <jspecify.version>1.0.0</jspecify.version>
     <junit-jupiter.version>5.10.3</junit-jupiter.version>
     <nullaway.version>0.11.0</nullaway.version>
     <parsson.version>1.1.6</parsson.version>
@@ -105,6 +106,11 @@
         <version>${jetbrains-annotations.version}</version>
       </dependency>
       <dependency>
+        <groupId>org.jspecify</groupId>
+        <artifactId>jspecify</artifactId>
+        <version>${jspecify.version}</version>
+      </dependency>
+      <dependency>
         <groupId>org.apiguardian</groupId>
         <artifactId>apiguardian-api</artifactId>
         <version>${apiguardian.version}</version>