br: reword 'Base64-encoded storage' for accurate machine translation

[qiancai]

What is changed, added or deleted? (Required)

This PR rewords ambiguous English phrases in br/br-compact-log-backup.md to produce more accurate machine translations to Japanese.

Problem:
The original English text contains the phrase "Base64-encoded storage", which machine translators (and human translators) interpret as "storage itself being Base64-encoded". This leads to unnatural Japanese translations like "Base64エンコードストレージ" (Base64-encoded storage), which is confusing because what is actually encoded is the storage URI/string, not the storage.

The root cause traces back to the Chinese source phrase "有了存储的 Base64" (the Base64 of the storage / the storage's Base64), which was translated to English as "Base64-encoded storage" — reversing the possessive relationship and changing the meaning.

Changes:

Before	After	Why
"matches the storage output"	"should match the output"	"storage output" is a confusing compound noun that MT renders literally as "storage出力"
"With the Base64-encoded storage, you can initiate the compaction..."	"Once you have the Base64-encoded string from the previous step, you can start compaction..."	Clarifies that the string (not the storage) was encoded; "initiate the compaction" is also overly formal
"the Base64-encoded storage string obtained earlier"	"the Base64-encoded string obtained in the previous step"	Same compound-noun ambiguity as above

These changes ensure machine translation produces natural Japanese: e.g., "Base64エンコードされた文字列" instead of the nonsensical "Base64エンコードストレージ".

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

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)

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

• This PR is translated from: br: reword 'Base64-encoded storage' for accurate machine translation docs#23047
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[github-actions]

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Source PR: pingcap/docs#23047
Target PR: #21714

Chinese documentation has been updated based on English documentation changes.

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