Use `variant="subtle"` for close icon on Dismissible Container

What does this MR do and why?

Use variant="subtle" for close icon on Dismissible Container instead of fixed color.

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

(note: there is no difference)

Before	After

How to set up and validate locally

1. Check out this branch
2. apply this patch:

diff --git a/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue b/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue
index ca20f90e9938..0e496ebce470 100644
--- a/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue
+++ b/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue
@@ -514,7 +514,6 @@ export default {
       </mr-widget-alert-message>
     </header>
     <mr-widget-suggest-pipeline
-      v-if="shouldSuggestPipelines"
       :pipeline-path="mr.mergeRequestAddCiConfigPath"
       :pipeline-svg-path="mr.pipelinesEmptySvgPath"
       :human-access="formattedHumanAccess"

3. Visit any MR
4. Because of the patch you applied, you will see the "Looks like there's no pipeline here" message, with a dismissible container around it. The x icon has class gl-fill-icon-subtle.

Related to #474920 (closed), #452231 (closed)