Update Cloudflares RSA root in the 2017 tutorial of setting up Gitlab Pages with Cloudflare.

Why is this change being made?

This merge request updates the Cloudflares RSA Root in this blog post from 2017

The reason i am submitting this merge request is because i still pretty often refer to this blog post when setting up a new website with gitlab pages.

When i was doing this today, gitlab showed me the error "Certificate misses intermediates". After being confused for a bit i figured out that the certificate shown in the post expired on 14 November. The new one is valid until 15 August 2029.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what