Update docs for Fleet Dashboard compute min export that it relies on Clickhouse
Problem to solve
A user recently provided feedback that they didn't know why the compute min export wasn't appearing on their Fleet Dashboard after installing it on self-managed. Currently, our documentation https://docs.gitlab.com/16.11/ee/ci/runners/runner_fleet_dashboard.html#export-compute-minutes-used-by-instance-runners does not explain that this feature relies on Clickhouse.
Further details
Proposal
Add a clarifier that Clickhouse is needed.