Merge branch 'use-sh-instead-of-bash' into 'ruby3-sync'

Use sh instead of bash for scripts/slack

See merge request gitlab-org/gitlab!97249

scripts/slack

-#!/bin/bash
+#!/bin/sh
 # This is based on https://gitlab.com/gitlab-org/gitlab-qa/-/blob/master/bin/slack
 #
 # Sends Slack notification MSG to CI_SLACK_WEBHOOK_URL (which needs to be set).