dt: Increase message size in MPT TS warmup

[StephanDollberg]

This speeds up the TS warmup which is one of the major contributors time
wise with increasing partition count.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v24.3.x
• v24.2.x
• v24.1.x

Release Notes

• none

[StephanDollberg]

btw if the above comment makes sense to anyone please let me know. I am struggling to parse it.

[vbotbuildovich]

Retry command for Build#60531

please wait until all jobs are finished before running the slash command

/ci-repeat 1
tests/rptest/tests/delete_records_test.py::DeleteRecordsTest.test_delete_records_concurrent_truncations@{"cloud_storage_enabled":true,"truncate_point":"start_offset"}

[vbotbuildovich]

CI test results

test results on build#60531

test_id	test_kind	job_url	test_status	passed
rm_stm_tests_rpunit.rm_stm_tests_rpunit	unit	https://buildkite.com/redpanda/redpanda/builds/60531#01944c68-15a7-4abe-9aba-bce86a689633	FLAKY	1/2
rm_stm_tests_rpunit.rm_stm_tests_rpunit	unit	https://buildkite.com/redpanda/redpanda/builds/60531#01944c68-15a7-4dc3-bee1-c8519536d9af	FLAKY	1/2
rptest.tests.delete_records_test.DeleteRecordsTest.test_delete_records_concurrent_truncations.cloud_storage_enabled=True.truncate_point=start_offset	ducktape	https://buildkite.com/redpanda/redpanda/builds/60531#01944cc2-54a3-4ac1-a177-b8e8b41d4450	FAIL	0/1