Remove @Beta from two methods in Throwables.

The methods, `getCausalChain` and `getCauseAs`, have existed for years. They work reliably and we are unlikely to change their APIs.

The remaining two `@Beta` methods, `lazyStackTrace` and `lazyStackTraceIsLazy`, should probably be deprecated in a later step. The optimization they encapsulate no longer works on JDK versions ≥ 9 and has never worked on Android. Meanwhile, JDK 9 introduced the `StackWalker` API which does work and covers the most usual use case of `lazyStackTrace`.

RELNOTES=The `Throwables` methods `getCausalChain` and `getCauseAs` were `@Beta` but are now fully supported.
PiperOrigin-RevId: 422652352

android/guava/src/com/google/common/base/Throwables.java

@@ -290,7 +290,6 @@ public static Throwable getRootCause(Throwable throwable) {
    * @return an unmodifiable list containing the cause chain starting with {@code throwable}
    * @throws IllegalArgumentException if there is a loop in the causal chain
    */
-  @Beta // TODO(kevinb): decide best return type
   public static List<Throwable> getCausalChain(Throwable throwable) {
     checkNotNull(throwable);
     List<Throwable> causes = new ArrayList<>(4);
@@ -330,7 +329,6 @@ public static List<Throwable> getCausalChain(Throwable throwable) {
    *     ClassCastException}'s cause is {@code throwable}.
    * @since 22.0
    */
-  @Beta
   @GwtIncompatible // Class.cast(Object)
   @CheckForNull
   public static <X extends Throwable> X getCauseAs(

guava/src/com/google/common/base/Throwables.java

@@ -290,7 +290,6 @@ public static Throwable getRootCause(Throwable throwable) {
    * @return an unmodifiable list containing the cause chain starting with {@code throwable}
    * @throws IllegalArgumentException if there is a loop in the causal chain
    */
-  @Beta // TODO(kevinb): decide best return type
   public static List<Throwable> getCausalChain(Throwable throwable) {
     checkNotNull(throwable);
     List<Throwable> causes = new ArrayList<>(4);
@@ -330,7 +329,6 @@ public static List<Throwable> getCausalChain(Throwable throwable) {
    *     ClassCastException}'s cause is {@code throwable}.
    * @since 22.0
    */
-  @Beta
   @GwtIncompatible // Class.cast(Object)
   @CheckForNull
   public static <X extends Throwable> X getCauseAs(