i18n(ja): translate the leftover English header cell in smooth-upgrade-tidb

[yahonda]

What is changed, added or deleted? (Required)

In smooth-upgrade-tidb.md, the version-compatibility table header row was partially translated: three cells were Japanese (オリジナル版 / アップグレード方法 / 注記) but "Upgraded version" was left in English. Translate it to アップグレード後のバージョン so the whole header row is in Japanese and consistent (the English source is "Original version | Upgraded version | Upgrade methods | Note"). One line.

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

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 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)

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

• This PR is translated from: N/A (Japanese-only fix; no English source change)
• Other reference link(s): part of the i18n-ja-release-8.5 machine-translation cleanup series

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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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