release-25.4: roachtest: reduce the number of concurrent works in tpcc/create

[jeffswenson]

Backport 1/1 commits from #159175.

/cc @cockroachdb/release

Fixes: #166164
Informs: #162008

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The tpcc/create tests is a bit of a stress test for the kv writer and it's an even bigger challenge for the crud writer. In steady state, the kv writer uses ~40% cpu to replicate the workload and the crud writer uses ~60% cpu. But this test isn't checking the steady state, its building up a backlog while the initial scan runs. So the workload needs to catch up using the remaining CPU headroom, which is what causes this test to fail due to high replication latency.

Now, the test uses 1/2 the warehouse count when running the workload. This lets us use a large data load when testing the create replicated table statement and produces a manageable backlog for the crud writer to catch up on.

Release note: none
Fixes: #156185
Part of: #158439

Release justification: test only change.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

✅ PR #166165 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The PR changes are limited to files under pkg/cmd/roachtest/, which is designated as a non-production file path in the CockroachDB Backport Policy. The changes are only applied to the test-related code, specifically the roachtest files that handle testing configurations and parameters for load and replication tests. This type of change falls under the exception for non-production and test-only changes. Additionally, the PR body includes 'Release justification: test only change,' which aligns with the policy exception for release justification. Considering these points, the PR meets the requirements and is exempt from the standard backport policy criteria.

✅ ENGREQ Check Passed: No ENGREQ required (non-production code or serious issues).

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[jeffswenson]

Thanks for the review!