i18n(ja): unify the Japanese term for Request Capacity Unit (RCU)

[yahonda]

What is changed, added or deleted? (Required)

The RCU term ("Request Capacity Unit") was rendered several ways in the Japanese docs — リクエスト容量ユニット, リクエスト容量単位, and 要求容量ユニット — alongside the intended fixed katakana compound リクエストキャパシティユニット.

This PR unifies the 13 容量-family occurrences → リクエストキャパシティユニット across 10 files, matching the established compound (per the house rule: capacity is normally 容量, but the RCU proper term is the fixed katakana リクエストキャパシティユニット).

Left unchanged (verified):

• The bare acronym RCU and the （RCU） glosses.
• Generic 容量 (disk/memory capacity): count went 573 → 560, i.e. exactly the 13 RCU conversions and nothing else.

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

• master (the latest development version)
• 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)
• v5.4 (TiDB 5.4 versions)

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

N/A

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

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[LGTM Timeline notifier]

Timeline:

• 2026-08-05 12:24:38.197856649 +0000 UTC m=+2617264.233951705: ☑️ agreed by qiancai.