move compress.grayscale.label to section #compress (#3048)

# Description of Changes

Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

src/main/resources/messages_ja_JP.properties

@@ -951,6 +951,7 @@ fileToPDF.submit=PDFを変換
 compress.title=圧縮
 compress.header=PDFを圧縮
 compress.credit=本サービスはPDFの圧縮/最適化にqpdfを使用しています。
+compress.grayscale.label=圧縮にグレースケールを適用する
 compress.selectText.1=手動モード - 1から9
 compress.selectText.1.1=最適化レベル6～9では、一般的なPDF圧縮に加えて画像解像度が縮小され、ファイルサイズがさらに縮小されます。レベルが高くなると、画像圧縮が強化され (元のサイズの最大 50%)、サイズはさらに縮小されますが、画像の品質が低下する可能性があります。
 compress.selectText.2=品質レベル:
@@ -1384,4 +1385,3 @@ validateSignature.cert.version=バージョン
 validateSignature.cert.keyUsage=キーの使用法
 validateSignature.cert.selfSigned=自己署名
 validateSignature.cert.bits=ビット
-compress.grayscale.label=圧縮にグレースケールを適用する