fix(export): make api export compatible with 1.20

fix gravitee-io/issues#2293

src/management/api/portal/general/dialog/apiExport.dialog.html

@@ -23,6 +23,7 @@ <h4>Export API definition as :</h4>
         <div layout="column" layout-align="space-between center">
           <md-select ng-model="data.exportVersion" placeholder="Export as">
             <md-option value="default" selected>Current version ({{ graviteeVersion }})</md-option>
+            <md-option value="1.20">1.20.x version</md-option>
             <md-option value="1.15">1.15.x to 1.16.x version</md-option>
           </md-select>
         </div>