Update alert boxes to use simpler format

What does this MR do?

We can now use Markdown alerts in the GitLab docs instead of Hugo shortcodes. We should switch to this style as it's simpler, more compact, easier to maintain, and works in /help (a rare /help win).

This MR uses regex find and replace with capture groups, looking only for "one-liners" (alert boxes with a single line of text), because it was simplest:

• Find: \{\{< alert type="([^"]*?)" >\}\}\n\n([^\n]*)\n\n\{\{< /alert >\}\}
  • First capture group ([^"]*?): The alert type (note, warning, etc)
  • Second capture group ([^\n]*): The alert text (one line).
• Replace: > [!$1]\n> $2

For example:

• Before:

  {{< alert type="note" >}}
  
  If Sidekiq is configured on a different server to the Rails server, you must add the LDAP configuration to every Sidekiq server as well for LDAP synchronisation to work.
  
  {{< /alert >}}

• After:

  > [!note]
  > If Sidekiq is configured on a different server to the Rails server, you must add the LDAP configuration to every Sidekiq server as well for LDAP synchronisation to work.

Related issues

• Related to Migrate to Hugo's alert format (gitlab-org/technical-writing/docs-gitlab-com#409)

Author's checklist

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