Migrate `GlDeprecatedSkeletonLoading` to `GlSkeletonLoader`

What does this MR do and why?

Related to #244669 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Short-circuit the condition to force the loading state to show up:

   diff --git a/app/assets/javascripts/ide/components/ide_side_bar.vue b/app/assets/javascripts/ide/components/ide_side_bar.vue
   index f32d35bf774..beedc078879 100644
   --- a/app/assets/javascripts/ide/components/ide_side_bar.vue
   +++ b/app/assets/javascripts/ide/components/ide_side_bar.vue
   @@ -35,7 +35,7 @@ export default {
        side="left"
        class="multi-file-commit-panel flex-column"
      >
   -    <template v-if="loading">
   +    <template v-if="true">
          <div class="multi-file-commit-panel-inner" data-testid="ide-side-bar-inner">
            <div v-for="n in 3" :key="n" class="multi-file-loading-container">
              <gl-skeleton-loader />

2. Open the Web IDE in any project.

MR acceptance checklist

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