Mark the GWT RegularImmutableBiMap.inverse as transient.

This eliminates a GWT-RPC warning.

The warning is basically irrelevant, since we haven't supported GWT-RPC in years, but it's nice to make it go away.

Fixes #8168

Closes #8177

RELNOTES=n/a
PiperOrigin-RevId: 861875508

Author: cpovirk

guava-gwt/src-super/com/google/common/collect/super/com/google/common/collect/RegularImmutableBiMap.java

@@ -29,7 +29,7 @@ final class RegularImmutableBiMap<K, V> extends ImmutableBiMap<K, V> {
   static final RegularImmutableBiMap<Object, Object> EMPTY =
       new RegularImmutableBiMap<Object, Object>();
 
-  private final ImmutableBiMap<V, K> inverse;
+  private final transient ImmutableBiMap<V, K> inverse;
 
   @SuppressWarnings("unchecked") // used only for the empty map, which works for any types
   RegularImmutableBiMap() {