doc: Update more links renaming Enablement

To suggest a change to the Product Handbook, please review the updating the Product Handbook section.

Related to:

gitlab-org/enablement-section/discussions#117

Please provide a brief explanation for this change:

(If there's a due date for merging this MR, be sure to include it here.)

• Enablement got renamed to Core Platform.
• While I'm here, update some stale links to Gitaly as Gitaly got split into Gitaly Cluster and Gitaly Git a while back, and moved to Enablement Core Platform.
• I have no idea what the "issues" in https://about.gitlab.com/direction/kickoff/#enablement-section are supposed to be, how and where they come from, but in any case the Enablement link is already broken -- so I just removed that inclusion. If you know more, we can fix it instead 😄

Please indicate the types of revisions being suggested for the Product Handbook (please check all that apply):

• Small improvement (typos, clarifications, etc.)
• Adding a new section
• Modifying existing section
• Documenting a new process
• Adding a new page or directory
• Other

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