admission,server: tune admission control based on tpcc, kv experiments

[cockroach-teamcity]

Exalate commented:

cockroachdb/cockroach#69311 --- Release note (ops change): New cluster settings admission.l0_sub_level_count_overload_threshold and admission.l0_file_count_overload_threshold can be used to tune admission control. Release justification: Low-risk update to new functionality.

Jira Issue: DOC-1461