[9.18] fix: ci: Update DNS Shotgun parameters for an updated dataset

We've switched to an updated dataset for shotgun jobs. The change in
underlying traffic caused the more sensitive doh-get (and partially dot)
jobs to overload the resolver, making the jobs unstable and unreliable,
due to an increased number of timeouts.

Readjust the load parameters slightly to avoid exceeding ~2 % of
timeouts in the cold cache scenario to stabilize the job results.

Backport of MR !10841

Merge branch 'backport-nicki/ci-shotgun-load-new-dataset-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10857