i18n(ja): use the English product name TiDB Cloud Serverless in 2025 release notes

[yahonda]

What is changed, added or deleted? (Required)

Restore the English product name TiDB Cloud Serverless where the machine translation had katakana-ized it as TiDB Cloudサーバーレス (and サーバーレスクラスター) in the 2025 TiDB Cloud release notes. 14 occurrences in tidb-cloud/releases/release-notes-2025.md.

The English source keeps "TiDB Cloud Serverless" in these historical release entries — the product was later renamed to TiDB Cloud Starter (the rename itself is announced in the same release notes, and the links already point to the #starter anchor), but the release notes deliberately preserve the name in use at the time of each release, matching the English source. So this PR restores Serverless (English), not Starter, to stay 1:1 with the English source.

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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