test/rgw/notifications: allow http server to buffer more requests

[yuvalif]

python doc:
https://docs.python.org/3/library/socketserver.html#socketserver.ThreadingMixIn say tha the default is 5

Fixes: https://tracker.ceph.com/issues/66033

Checklist

• Tracker (select at least one)
  • References tracker ticket
  • Very recent bug; references commit where it was introduced
  • New feature (ticket optional)
  • Doc update (no ticket needed)
  • Code cleanup (no ticket needed)
• Component impact
  • Affects Dashboard, opened tracker ticket
  • Affects Orchestrator, opened tracker ticket
  • No impact that needs to be tracked
• Documentation (select at least one)
  • Updates relevant documentation
  • No doc update is appropriate
• Tests (select at least one)
  • Includes unit test(s)
  • Includes integration test(s)
  • Includes bug reproducer
  • No tests

Show available Jenkins commands

• jenkins retest this please
• jenkins test classic perf
• jenkins test crimson perf
• jenkins test signed
• jenkins test make check
• jenkins test make check arm64
• jenkins test submodules
• jenkins test dashboard
• jenkins test dashboard cephadm
• jenkins test api
• jenkins test docs
• jenkins render docs
• jenkins test ceph-volume all
• jenkins test ceph-volume tox
• jenkins test windows
• jenkins test rook e2e

[yuvalif]

according to these runs:

• https://pulpito.ceph.com/yuvalif-2024-05-23_05:50:06-rgw:notifications-wip-yuval-kafka-cleanup-distro-default-smithi/
• https://pulpito.ceph.com/yuvalif-2024-05-23_07:59:11-rgw:notifications-wip-yuval-kafka-cleanup-distro-default-smithi/
• https://pulpito.ceph.com/yuvalif-2024-05-23_11:43:25-rgw:notifications-wip-yuval-kafka-cleanup-distro-default-smithi/
• https://pulpito.ceph.com/yuvalif-2024-05-25_14:50:24-rgw:notifications-wip-yuval-test-http-distro-default-smithi/
• https://pulpito.ceph.com/yuvalif-2024-05-26_09:24:35-rgw:notifications-wip-yuval-test-http-distro-default-smithi/

this fixes test_notification_push_http

however, note that other http related failures are still observed:
test_persistent_ps_s3_data_path_v2_migration
test_ps_s3_lifecycle_on_master
test_ps_s3_persistent_topic_stats
test_persistent_ps_s3_reload