v8.5.5 br: remove outdated PITR limitation (#21238)#21239

ti-chi-bot[bot] merged 2 commits intopingcap:feature/preview-v8.5.5pingcap/docs-cn:feature/preview-v8.5.5from

ti-chi-bot:cherry-pick-21238-to-release-8.5ti-chi-bot/docs-cn:cherry-pick-21238-to-release-8.5Copy head branch name to clipboard