Add MIT OS License to Pricing Page

Why is this change being made?

During Public Stream | Founder-to-founder OSS advice, Sid requested that we add that the Free tier is open source.

Added:

• On https://about.gitlab.com/pricing/,
• Free column -> Under 400 CI....,
• add Open Source - MIT License,
• and then make it a clickable link to open https://gitlab.com/gitlab-org/gitlab-foss/-/blob/master/LICENSE in a new window.
• Called out

For GitLab Self-Managed installations, not applicable for GitLab SaaS

https://gitlab.com/gitlab-com/ceo-shadow/tasks/-/issues/216

Author Checklist

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