Replace button GlButton

What does this MR do?

Updating button, per: #229316 (closed)

Screenshots

Current	WIP: :hover display

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #229316 (closed)

ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/approvals_footer.vue

 <script>
-import { GlDeprecatedButton, GlLoadingIcon, GlIcon } from '@gitlab/ui';
+import { GlButton, GlDeprecatedButton } from '@gitlab/ui';
 import { __ } from '~/locale';
 import UserAvatarList from '~/vue_shared/components/user_avatar/user_avatar_list.vue';
 import ApprovalsList from './approvals_list.vue';
 
 export default {
   components: {
-    GlIcon,
+    GlButton,
     GlDeprecatedButton,
-    GlLoadingIcon,
     UserAvatarList,
     ApprovalsList,
   },
@@ -55,6 +54,9 @@ export default {
     suggestedApproversTrimmed() {
       return this.suggestedApprovers.slice(0, Math.min(5, this.suggestedApprovers.length));
     },
+    shouldShowLoadingSpinner() {
+      return !this.isCollapsed && this.isLoadingRules;
+    },
   },
   methods: {
     toggle() {
@@ -67,15 +69,16 @@ export default {
 <template>
   <div>
     <div class="mr-widget-extension d-flex align-items-center pl-3">
-      <button
-        class="btn btn-blank square s32 gl-mr-3"
-        type="button"
+      <!-- TODO: simplify button classes once https://gitlab.com/gitlab-org/gitlab-ui/-/issues/1029 is completed -->
+      <gl-button
+        class="gl-mr-3"
+        :class="{ 'gl-shadow-none!': shouldShowLoadingSpinner }"
         :aria-label="ariaLabel"
+        :loading="shouldShowLoadingSpinner"
+        :icon="angleIcon"
+        category="tertiary"
         @click="toggle"
-      >
-        <gl-loading-icon v-if="!isCollapsed && isLoadingRules" />
-        <gl-icon v-else :name="angleIcon" :size="16" />
-      </button>
+      />
       <template v-if="isCollapsed">
         <user-avatar-list :items="suggestedApproversTrimmed" :breakpoint="0" empty-text="" />
         <gl-deprecated-button variant="link" @click="toggle">{{