feat(GlSingleStat): Change px to rem

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mentioned in merge request gitlab!142582 (closed)

mentioned in merge request gitlab!142686 (merged)

mentioned in merge request gitlab-docs!4555 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3713 (merged)

mentioned in merge request gitlab!143271 (merged)

mentioned in merge request gitlab!143284 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!268 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!269 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!21 (merged)