Fix outdated statement about TiDB's default transaction mode in optimistic-transaction.md (#21007)#21011

ti-chi-bot[bot] merged 6 commits into

pingcap:release-8.5pingcap/docs-cn:release-8.5from

ti-chi-bot:cherry-pick-21007-to-release-8.5ti-chi-bot/docs-cn:cherry-pick-21007-to-release-8.5Copy head branch name to clipboard