Create an ad iterator over ads that support multiple text, image, and video
assets by using the condition `Type = MULTI_ASSET_RESPONSIVE_DISPLAY_AD`. An ad
iterator with condition `Type = RESPONSIVE_DISPLAY_AD` will only iterate
over legacy ads because this condition is deprecated. Update existing code to
explicitly use `Type = LEGACY_RESPONSIVE_DISPLAY_AD`.

The following snippet illustrates some of the API differences and similarities
between the two types of responsive display ads. Read the
[Responsive Display Ad reference](https://developers.google.com/google-ads/scripts/docs/reference/adsapp/adsapp_responsivedisplayad)
for more details.

        // let legacyRdaIterator = AdsApp.ads().withCondition("Type = RESPONSIVE_DISPLAY_AD").get();
        let legacyRdaIterator = AdsApp.ads().withCondition("Type = LEGACY_RESPONSIVE_DISPLAY_AD").get();
        while (legacyRdaIterator.hasNext()) {
          let responsiveDisplayAd = legacyRdaIterator.next().asType().responsiveDisplayAd();

          // Legacy responsive display ads have just one long headline.
          let longHeadline = responsiveDisplayAd.getLongHeadline();

          // And they have one short headline, too.
          let shortHeadline = responsiveDisplayAd.getShortHeadline();

          // This call to .getHeadlines() returns null, because the method is not
          // meaningful when called on a legacy responsive display ad.
          assert(responsiveDisplayAd.getHeadlines() === null);

          // ... etc. ...
        }

        let rdaIterator = AdsApp.ads().withCondition("Type = MULTI_ASSET_RESPONSIVE_DISPLAY_AD").get();
        while (rdaIterator.hasNext()) {
          let responsiveDisplayAd = rdaIterator.next().asType().responsiveDisplayAd();

          // Responsive display ads have just one long headline.
          let longHeadline = responsiveDisplayAd.getLongHeadline();

          // But they can have multiple short headline text assets.
          let shortHeadlineAssets = responsiveDisplayAd.getHeadlines();

          // This call to .getShortHeadline() returns null, because the method is only
          // meaningful when called on a legacy responsive display ad.
          assert(responsiveDisplayAd.getShortHeadline() === null);

          // ... etc. ...
        }

        let rdaAndLegacyIterator = AdsApp.ads().withCondition("Type IN [LEGACY_RESPONSIVE_DISPLAY_AD, MULTI_ASSET_RESPONSIVE_DISPLAY_AD]").get();
        while (rdaAndLegacyIterator.hasNext()) {
          let responsiveDisplayAd = legacyRdaIterator.next().asType().responsiveDisplayAd();
          let longHeadline = responsiveDisplayAd.getLongHeadline();

          // The .isLegacy() method can be used to differentiate between responsive
          // display ad types within an iterator over both.
          if (responsiveDisplayAd.isLegacy()) {
            let shortHeadline = responsiveDisplayAd.getShortHeadline();
            // ... etc. ...
          } else {
            let shortHeadlineAssets = responsiveDisplayAd.getHeadlines();
            // ... etc. ...
          }
        }