Switch to use gl-alert instead of createFlash

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The following discussion from !57442 (merged) should be addressed:

• @svedova started a discussion: (+2 comments)

  question we should prefer using gl-alert over createFlash. Also, the test for the error state is missing 🙈

• https://gitlab.com/gitlab-org/gitlab/blob/3273c9659767748756b515c530fc996b2529c3a8/app/assets/javascripts/projects/compare/components/revision_dropdown.vue#L97

• https://gitlab.com/gitlab-org/gitlab/blob/3273c9659767748756b515c530fc996b2529c3a8/app/assets/javascripts/projects/compare/components/revision_dropdown.vue#L76