Elasticsearch: Add support for template variable in date histogram min_doc_count (#21064)

Fixes #21052

public/app/plugins/datasource/elasticsearch/partials/bucket_agg.html

@@ -65,7 +65,7 @@
     <div class="gf-form offset-width-7">
       <label class="gf-form-label width-10">Min Doc Count</label>
       <input
-        type="number"
+        type="text"
         class="gf-form-input max-width-12"
         ng-model="agg.settings.min_doc_count"
         ng-blur="onChangeInternal()"