snap_backup: abort last connection of preparing while there are many (#16388)

[ti-chi-bot]

This is an automated cherry-pick of #16388

What is changed and how it works?

Issue Number: Close #16382

What's Changed:

Now, a newly established prepare disk snapshot backup stream will abort the former one.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Release note

None

[ti-chi-bot]

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[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be approved by the approvers firstly.
2. AFTER it has been approved by approvers, please wait for the cherry-pick merging approval from triage owners.

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