Update Load Balancer sizing in Ref Arch docs

What does this MR do?

MR updates sizing guidance for Load Balancers to move up a size in the Reference Architecture documentation after a review of CPU and Network Bandwidth usage in heavy situations such as a large amount of concurrent clones of a large monorepo. Additionally, for AWS machine types specifically network bursting can be impactful compared with others so machine types have been switched to network enhanced variants (c5n) to compensate.

Related issues

Closes gitlab-org/quality/reference-architectures#286 (closed)

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