Change how updating IGs to an unknown executionMode behaves. (#927)

When joining IGs with unrecognized modes, we use "compatibility" for forward compatibility.  Seems like we should do the same for updates.

Note that neither this behavior nor the originally described behavior are what currently happens - we currently ignore unrecognized modes.  Want to get approval for this before changing behvaior.

spec.bs

@@ -3566,7 +3566,7 @@ The <dfn for=Navigator method>updateAdInterestGroups()</dfn> method steps are:
         <dd>
         1. If |value| is "`compatibility`", "`frozen-context`", or "`group-by-origin`",
           set |ig|'s [=interest group/execution mode=] to |value|.
-        1. Otherwise, jump to the step labeled <i><a href=#abort-update>Abort update</a></i>.
+        1. Otherwise, set |ig|'s [=interest group/execution mode=] to "`compatibility`".
 
         <dt>"`biddingLogicURL`"
         <dt>"`biddingWasmHelperURL`"