bench: lower attach concurrency

it has been observed that we get transaction serialization issues
otherwise.

evidence: https://neon-github-public-dev.s3.amazonaws.com/reports/main/10402057346/index.html#suites/c62b105f3a4f00dd6be4ad88810e0e02/7c03011d9dbd5ec4/

test_runner/fixtures/pageserver/many_tenants.py

@@ -65,7 +65,7 @@ def single_timeline(
             override_storage_controller_generation=True,
         )
 
-    with concurrent.futures.ThreadPoolExecutor(max_workers=22) as executor:
+    with concurrent.futures.ThreadPoolExecutor(max_workers=1) as executor:
         executor.map(attach, tenants)
 
     # Benchmarks will start the pageserver explicitly themselves