Fix bottom popover arrow color

What does this MR do and why?

Scopes a bottom popover arrow color change to the duo_chat_callout.vue component by adding a new duo-chat-callout-popover class to the GlPopover child component. Due to lack of specificity, the color change was being applied to all bottom popovers site-wide.

Resolves #455939 (closed).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Component	Before	After
Duo Chat Callout (no change)
User Achievements

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. If you've already dismissed the Duo Chat callout at the top right, apply the patch below and verify that the popover arrow remains purple.
2. Navigate to your user profile (http://${YOUR_LOCAL_INSTANCE}/${YOUR_USERNAME}), hover over an achievement and verify that the popover's arrow is the same as its body.

Duo Chat Callout Patch

diff --git a/ee/app/assets/javascripts/ai/components/global_callout/duo_chat_callout.vue b/ee/app/assets/javascripts/ai/components/global_callout/duo_chat_callout.vue
index 44855cc57bad..668d847f43dd 100644
--- a/ee/app/assets/javascripts/ai/components/global_callout/duo_chat_callout.vue
+++ b/ee/app/assets/javascripts/ai/components/global_callout/duo_chat_callout.vue
@@ -70,9 +70,9 @@ export default {
   <user-callout-dismisser v-if="popoverTarget" feature-name="duo_chat_callout">
     <template #default="{ dismiss, shouldShowCallout }">
       <gl-popover
-        v-if="shouldShowCallout"
+        v-if="true"
         :target="popoverTarget"
-        :show="shouldShowCallout"
+        :show="true"
         show-close-button
         :css-classes="['duo-chat-callout-popover', 'gl-max-w-48', 'gl-shadow-lg', 'gl-p-2']"
         triggers="manual"