Add a missing @since tag, and remove @FunctionalInterface from a package-private method.

(The latter prepares for [Android compatibility for Java 8 APIs](#6567).)

RELNOTES=n/a
PiperOrigin-RevId: 626483101

Author: cpovirk

guava-testlib/src/com/google/common/collect/testing/SpliteratorTester.java

@@ -45,7 +45,11 @@
 import java.util.function.Supplier;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
-/** Tester for {@code Spliterator} implementations. */
+/**
+ * Tester for {@code Spliterator} implementations.
+ *
+ * @since 21.0
+ */
 @GwtCompatible
 @ElementTypesAreNonnullByDefault
 public final class SpliteratorTester<E extends @Nullable Object> {

guava/src/com/google/common/collect/CollectSpliterators.java

@@ -309,7 +309,6 @@ abstract static class FlatMapSpliterator<
           OutSpliteratorT extends Spliterator<OutElementT>>
       implements Spliterator<OutElementT> {
     /** Factory for constructing {@link FlatMapSpliterator} instances. */
-    @FunctionalInterface
     interface Factory<InElementT extends @Nullable Object, OutSpliteratorT extends Spliterator<?>> {
       OutSpliteratorT newFlatMapSpliterator(
           @CheckForNull OutSpliteratorT prefix,