[Doc]: chore: increase OC sponsor count #1865 (#1876)

* cherry-pick vuejs/v2.vuejs.org@4189659

Author: oohira

themes/vue/layout/partials/sponsors.ejs

@@ -19,7 +19,7 @@
 <div class="open-collective-sponsors">
   <h3>OpenCollective スポンサー</h3>
   <h4>プラチナ</h4>
-  <%_ for (let i = 0; i < 2; i++) {_%>
+  <%_ for (let i = 0; i < 3; i++) {_%>
   <a
     href="https://opencollective.com/vuejs/tiers/platinum-sponsors/<%- i %>/website"
     rel="sponsored noopener"
@@ -29,7 +29,7 @@
   </a>
   <%_ } _%>
   <h4>ゴールド</h4>
-  <%_ for (let i = 0; i < 8; i++) {_%>
+  <%_ for (let i = 0; i < 20; i++) {_%>
   <a
     href="https://opencollective.com/vuejs/tiers/gold-sponsors/<%- i %>/website"
     rel="sponsored noopener"