Update gitlab_com.yml to clarify you need silver/gold to get more minutes

Merged Lee Matos requested to merge lm-update-faq-silver-gold-upgrade into master

ZD: https://gitlab.zendesk.com/agent/tickets/83092

A user was confused and upgraded to bronze thinking they'd get 2000 additional minutes. This FAQ update hopes to clarify that you need to go to silver or gold to get more minutes.