BR v8.5.5: update br compact backup

[Oreoxmt]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added, or deleted? (Required)

Update the TiDB version reference from "v9.0.0" to "v8.5.5".

Which TiDB Operator version(s) do your changes apply to? (Required)

• main (the latest development version for v2.x)
• v2.0 (TiDB Operator 2.0 versions)
• release-1.x (the latest development version for v1.x)
• v1.6 (TiDB Operator 1.6 versions)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
• v1.3 (TiDB Operator 1.3 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): br compact backup user document #2676

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[Oreoxmt]

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[Oreoxmt]

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