i18n(ja): fix mistranslated KILL heading and inconsistent high-concurrency term in release notes

[yahonda]

What is changed, added or deleted? (Required)

• privilege-management.md: restore the literal SQL statement name ### KILL {#kill}, which had been mistranslated as the verb 殺す ("to kill" as in murder). The body text and sql-statements/sql-statement-kill.md already use KILL as-is, so this heading was the odd one out.
• 6 release notes files (release-8.1.0.md, release-6.5.10.md, release-7.1.6.md, release-7.5.2.md, release-3.0.0-rc.3.md, release-2.1.13.md): fix 高同時読み取りシナリオ/高同時シナリオ to 高同時実行読み取りシナリオ/高同時実行シナリオ. These translate the English "high-concurrency read scenarios" / "high concurrent scenarios", and 高同時実行 (not bare 高同時) is the natural, standard translation for "high concurrency" used everywhere else in the docs.

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)

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

• This PR is translated from:
• Other reference link(s): i18n(ja): fix mistranslated function-name headings and misc defects #23512

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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Timeline:

• 2026-08-13 03:15:38.439954388 +0000 UTC m=+3275524.476049444: ☑️ agreed by qiancai.