Use JSpecify annotations in the public release.

We've been using these internally for over a year. With JSpecify 1.0 not far off and wider adoption to gradually follow, now seems like the time to expand our public usage of the annotations. We've already been using them in a few projects, but Truth may become the mostly widely used project that users actually link against to use them. At the same time, it's a library that's used only in testing code, so the stakes remain relatively low.

Most users will see no effect from this change, since most users don't use nullness checking and since we already used some nullness annotations in our public release. The main effect users are likely to see is if they pass nullable values for parameters that are now recognized as non-nullable. Under Kotlin, the effect should normally be a warning, not an error, at least until [Kotlin 2.1 or so](https://youtrack.jetbrains.com/issue/KT-55586/Handle-nullability-from-jspecify-annotations-properly#focus=Comments-27-8368666.0-0). Please still [report any problems](https://github.com/google/truth/issues/new).

(progress toward JSpecify adoption in our projects in general, including google/guava#2960)

RELNOTES=Added more nullness information to our APIs (in the form of [JSpecify](https://jspecify.dev/) annotations). This could lead to additional warnings (or even errors) for users of Kotlin and other nullness checkers. Please [report any problems](https://github.com/google/truth/issues/new).
PiperOrigin-RevId: 647012817

Author: cpovirk

core/pom.xml

@@ -15,8 +15,8 @@
       <artifactId>guava</artifactId>
     </dependency>
     <dependency>
-      <groupId>org.checkerframework</groupId>
-      <artifactId>checker-qual</artifactId>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
     </dependency>
     <dependency>
       <groupId>junit</groupId>

core/src/main/java/com/google/common/truth/AbstractArraySubject.java

@@ -20,13 +20,15 @@
 import static com.google.common.truth.Fact.simpleFact;
 
 import java.lang.reflect.Array;
-import org.checkerframework.checker.nullness.qual.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /**
  * A common supertype for Array subjects, abstracting some common display and error infrastructure.
  *
  * @author Christian Gruber (cgruber@israfil.net)
  */
+@NullMarked
 abstract class AbstractArraySubject extends Subject {
   private final @Nullable Object actual;