backup-manager: try to directly get the backup size from backupmeta before calculate the backup size (#5411)

[ti-chi-bot]

This is an automated cherry-pick of #5411

What problem does this PR solve?

The current backup-manager cannot parse the backupmetaV2.

Close #5407

What is changed and how does it work?

try to directly get the backup size from backupmeta before calculate the backup size. Notice that if the br supports backupemtaV2, it also supports to record the backup size into the backupmeta.

Code changes

• [x] Has Go code change
• [ ] Has CI related scripts change

Tests

• [ ] Unit test
• [ ] E2E test
• [ ] Manual test
• [ ] No code

Side effects

• [ ] Breaking backward compatibility
• [ ] Other side effects:

Related changes

• [ ] Need to cherry-pick to the release branch
• [ ] Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

[ti-chi-bot[bot]]

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[Leavrth]

blocked: need to update k8s package