Turn off protocol-level prepared statements

Details: see discussions in gitlab-org/gitlab!135196 (merged)

Slack: https://postgres-ai.slack.com/archives/CSXS2JV6W/p1698768352759449?thread_ts=1698767396.090009&cid=CSXS2JV6W

Plan cache makes it difficult to checking different plans via using set enable_xxx to 'off'