i18n(ja): fix translation defects in the v3.0.15 to v3.1.2 release notes

[yahonda]

What is changed, added or deleted? (Required)

Fixes translation defects in the Japanese release notes for v3.0.15 to v3.1.2 (10 files; v3.0.19, v3.1.0-beta, and v3.1.2 had none). This is the second half of round 8 of the releases/ review. I checked each item against its EN bullet (release-8.5) and against resources/terms-en-ja.md.

Fixes

• release-3.1.0-rc.md summary: the code spans were broken (「RECOVER」, 「集計」), and Aggregation was translated.
• 3.1.0-beta.1 L23: five monitoring metric names were shuffled through the sentence.
• 3.1.0-ga:
  • L69: runtime.growslice and chunk.column.nullBitMap were swapped.
  • L62/L66: truncated sentences.
  • L82: リージョン結合 → リージョンマージ.
• 3.0.16:
  • L17: the plural "Backoffers" became 「Backoffer秒」 ("seconds").
  • L39: 障害ポイント → failpoint.
  • L40: "the timeout of pre-splitting Regions".
• 3.0.20 L22: "Raise an error when preparing LOAD DATA" read as a new bug.
• 3.0.15:
  • "prepared manner" → プリペアド方式.
  • "expensive threshold" → expensive しきい値.
  • Leader stays in English.
• Capitalized Learner stays in English, and lowercase "learner" is ラーナー; 領域 → リージョン; ラフトストア → raftstore.
• Restores particles dropped after links or backticks, and the 「では」 after component names in summaries.

Not changed:

• Existing 「パニック」 and the established 「ラーナー」.
• Some Grafana dashboard names in v3.0.0-rc.1 and v3.0.2.
• The order and spacing of handles and issue links.

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• 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): i18n(ja): fix translation defects in the v8.5.x release notes #24143, i18n(ja): fix translation defects in the v8.1.0, v8.1.2, and v8.2.0 release notes #24148, i18n(ja): fix translation defects in the v8.0.0, v8.3.0, and v8.4.0 release notes #24152, i18n(ja): fix translation defects in the v7.0.0 to v7.4.0 release notes #24153, i18n(ja): fix translation defects in the v7.5.x and v7.6.0 release notes #24154, i18n(ja): fix translation defects in the v6.0.0 to v6.4.0 release notes #24157, i18n(ja): fix translation defects in the v6.5.x and v6.6.0 release notes #24158, i18n(ja): fix translation defects in the v5.0.x to v5.2.x release notes #24164, i18n(ja): fix translation defects in the v5.3.x and v5.4.x release notes and the release index #24165, i18n(ja): fix translation defects in the v4.0.0-beta to v4.0.9 release notes #24171, i18n(ja): fix translation defects in the v4.0.10 to v4.0.16 release notes #24172

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 a4dfec7 and aa7be80.

📒 Files selected for processing (10)

• releases/release-3.0.15.md
• releases/release-3.0.16.md
• releases/release-3.0.17.md
• releases/release-3.0.18.md
• releases/release-3.0.20.md
• releases/release-3.1.0-beta.1.md
• releases/release-3.1.0-beta.2.md
• releases/release-3.1.0-ga.md
• releases/release-3.1.0-rc.md
• releases/release-3.1.1.md

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