Use @NullMarked instead of our custom @ElementTypesAreNonnullByDefault annotations.

This is the next step toward [using JSpecify in Guava](jspecify/jspecify#239 (comment)). At the end of that path, we'll be able to [remove our dependency on JSR-305](#2960) (and on the Checker Framework's annotations), and we'll have one less blocker to [providing a `module-info`](#2970).

`@NullMarked` allows tools like kotlinc to produce errors for code like `ImmutableList<String?>`. (Before releasing this change, I'll conduct some further testing to more fully characterize the effects, both under Kotlin 2.1 and prior.) As we make further changes, it will allow kotlinc to detect even more nullness problems. We will make these changes in a series of incremental releases so that users can pick them up gradually, as we did inside Google. In simple cases, users may wish to pick up all the changes at once instead by upgrading straight from Guava 33.4.0 (or an earlier version) to Guava 33.4.4 (or whatever the version to make the final changes ends up being).

RELNOTES=Replaced our custom `@ElementTypesAreNonnullByDefault` annotations with the JSpecify `@NullMarked` annotation.
PiperOrigin-RevId: 708598410

Author: cpovirk

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

@@ -19,6 +19,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.util.Collection;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.junit.Ignore;
 
@@ -32,7 +33,7 @@
 @Ignore("test runners must not instantiate and run this directly, only via suites we build")
 // @Ignore affects the Android test runner, which respects JUnit 4 annotations on JUnit 3 tests.
 @SuppressWarnings("JUnit4ClassUsedInJUnit3")
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public abstract class AbstractCollectionTester<E extends @Nullable Object>
     extends AbstractContainerTester<Collection<E>, E> {
 

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

@@ -27,6 +27,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.junit.Ignore;
 
@@ -42,7 +43,7 @@
 @Ignore("test runners must not instantiate and run this directly, only via suites we build")
 // @Ignore affects the Android test runner, which respects JUnit 4 annotations on JUnit 3 tests.
 @SuppressWarnings("JUnit4ClassUsedInJUnit3")
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public abstract class AbstractContainerTester<C, E extends @Nullable Object>
     extends AbstractTester<OneSizeTestContainerGenerator<C, E>> {
   protected SampleElements<E> samples;