Rename domain experts to domain specialist

Why is this change being made?

Rename domain experts to domain specialist as expert tends to have the connotation that an individual knows it all. We also seem to define expert as someone with above average experience with a certain topic according to https://about.gitlab.com/company/team/structure/#expert. The term specialist seems to be a better fit and may make team members feel more comfortable adding themselves to the list.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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