Card sizing for security dashboard discovery moment experiment

What does this MR do and why?

Card sizing for security dashboard discovery moment experiment

Related to #365499

Screenshots or screen recordings

Before	After Large	After Medium	After Small

How to set up and validate locally

1. Act like SAAS https://docs.gitlab.com/ee/development/ee_features.html#act-as-saas
2. Enable the experiment flag bin/rails c => Feature.enable(:showcase_free_security_features)
3. Create a group GROUP and a project PROJECT
4. Go to the Security tab of the group and of the project

• http://gdk.test:3000/groups/GROUP/-/security/discover
• http://gdk.test:3000/GROUP/PROJECT/-/security/discover

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

ee/app/assets/javascripts/vue_shared/showcase/card_showcase.vue

@@ -87,7 +87,7 @@ export default {
       <div class="gl-bottom-0 gl-display-flex">
         <gl-button
           v-if="primaryAction"
-          class="gl-mr-2"
+          class="gl-mr-2 gl-mb-2"
           category="primary"
           variant="confirm"
           :href="primaryLink"
@@ -105,7 +105,7 @@ export default {
 
         <gl-button
           v-if="secondaryAction"
-          class="gl-mr-2"
+          class="gl-mr-2 gl-mb-2"
           category="secondary"
           variant="confirm"
           :href="secondaryLink"