Add certificate_based_clusters FF to Service Ping

What does this MR do and why?

Add the value of the certificate_based_clusters FF to Service Ping.

We want to be able to check how many self-managed instances have this FF true vs false. This will help us gauging the risks of removing the feature that's behind this FF on the next major milestone.

How to set up and validate locally

Simply run the command:

bundle exec rake gitlab:usage_data:generate | jq .settings.certificate_based_clusters_ff
true

One can also start rails c and disable the FF and then retry the above command, which should then return false:

Feature.disable(:certificate_based_clusters)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #353415 (closed)