gc (ticdc): optimize the algorithm calculating gc safepoint (#8464) #8887

…s, put the error changefeed into failed state and calculate the ticdc global gc safepoint based on checkpoint ts of all changefeeds and give users 24 hours grace period to handle the failed changefeed. e.g., Have two ChangeFeeds, cf1(failed) with checkpointTs ts1 cf2(normal) with checkpointTs ts2 the global gc safepoint will be: min(ts2, max(ts1, currentPDTs - 24 hours))

…lized