br: cherry pick pr 40504

[fengou1]

What problem does this PR solve?

cherry pick the pr 40504
Issue Number: close #40391 #41005

Problem Summary:

What is changed and how it works?

refactoring code to remove totalFlashbackRegions since task completion region already contains all regions

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

1. follow the doc https://docs.pingcap.com/zh/tidb-in-kubernetes/stable/backup-to-aws-s3-by-snapshot to do the backup
2. follow the doc https://docs.pingcap.com/zh/tidb-in-kubernetes/stable/restore-from-aws-s3-by-snapshot to do the restore
3. restore success

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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[ti-chi-bot]

@fengou1: PR needs rebase.

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