[9.20] chg: ci: Increase the load TCP/DoT shotgun perf tests

Due to the recent improvements to the TCP processing, much higher loads
can be handled by BIND9 without causing client timeouts. The updated
parameters give us useful data for both cold and hot cache testing.

Backport of MR !9494

Merge branch 'backport-nicki/increase-tcp-dot-shotgun-load-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9548