README: Update GitLab CI link
What does this MR do and why?
The link to the GitLab CI page is broken and results in an 404 page not found.
This MR changes the link to https://about.gitlab.com/solutions/continuous-integration/
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.