i18n(ja): translate sentences left untranslated in English

[yahonda]

First-time contributors' checklist

What is changed, added or deleted? (Required)

Three prose sentences/bullets were left in raw English amid otherwise fully translated Japanese text:

• releases/release-6.1.0.md: two bullets under the dynamic-config feature description and the upgrade-compatibility notes. The second one reuses the wording already translated for the same content appearing elsewhere in this same release note (lines 315-318), for consistency.
• security-compatibility-with-mysql.md: one numbered setup step for configuring tidb_auth_token.
• ticdc/ticdc-open-api.md: one lead-in sentence before the sink_config JSON example.

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

• i18n-ja-release-8.5 (TiDB Japanese documentation for TiDB 8.5 versions)

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

• This PR is translated from:
• Other reference link(s):

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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📥 Commits

Reviewing files that changed from the base of the PR and between 645c2ff and a6171db.

📒 Files selected for processing (3)

• releases/release-6.1.0.md
• security-compatibility-with-mysql.md
• ticdc/ticdc-open-api.md

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