set proper signature marker in mails

use rfc 3676 mail signature delimiters ("-- ") instead of the previous
"---" one.

closes #43617
Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>

app/views/layouts/mailer.text.erb

 <%= yield -%>
 
----
+-- <%# signature marker %>
 You're receiving this email because of your account on <%= Gitlab.config.gitlab.host %>.

app/views/layouts/notify.text.erb

 <%= yield -%>
 
----
+-- <%# signature marker %>
 <%  if @target_url -%>
 <%    if @reply_by_email -%>
 <%=     "Reply to this email directly or view it on GitLab: #{@target_url}" -%>