Remove isSettingFinding, closeSettingFindings and the sentinel_kept table once #2505 is closed #3488

#3314 removed the setting ask (POST /asks/setting, the home card, the terminal K/B, answerSetting, recordKept). What is left waits for the last open setting: finding (#2505) to close:

• remove isSettingFinding and its setting reason in src/crew/held.ts, and closeSettingFindings and its call in src/crew/sentinel.ts (so src/crew/setting_finding.ts goes)
• add a migration dropping sentinel_kept, once the migrations that created it are out of the two newest releases (the additive rule in src/core/releases.json)
• update the README paragraph on legacy setting findings

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.