Migrate from Checker Framework annotations to JSpecify annotations.

This is the next step toward [using JSpecify in Guava](jspecify/jspecify#239 (comment)).

RELNOTES=Migrated from Checker Framework annotations to JSpecify annotations.
PiperOrigin-RevId: 708707163

Author: cpovirk

Diff for: android/guava-testlib/src/com/google/common/collect/testing/AbstractCollectionTester.java

@@ -20,7 +20,7 @@
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.util.Collection;
 import org.jspecify.annotations.NullMarked;
-import org.checkerframework.checker.nullness.qual.Nullable;
+import org.jspecify.annotations.Nullable;
 import org.junit.Ignore;
 
 /**

Diff for: android/guava-testlib/src/com/google/common/collect/testing/AbstractContainerTester.java

@@ -28,7 +28,7 @@
 import java.util.Collection;
 import java.util.List;
 import org.jspecify.annotations.NullMarked;
-import org.checkerframework.checker.nullness.qual.Nullable;
+import org.jspecify.annotations.Nullable;
 import org.junit.Ignore;
 
 /**