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
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### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
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for more details.

src/main/resources/messages_ko_KR.properties

@@ -951,6 +951,7 @@ fileToPDF.submit=PDF로 변환
 compress.title=압축
 compress.header=PDF 압축
 compress.credit=이 서비스는 PDF 압축/최적화를 위해 qpdf를 사용합니다.
+compress.grayscale.label=압축을 위해 그레이스케일 적용
 compress.selectText.1=수동 모드 - 1에서 5
 compress.selectText.1.1=최적화 레벨 6에서 9에서는 일반적인 PDF 압축 외에도 이미지 해상도가 낮아져 파일 크기가 더욱 감소합니다. 높은 레벨은 더 강력한 이미지 압축(원본 크기의 최대 50%)을 초래하여 더 큰 크기 감소를 달성하지만 이미지 품질이 저하될 수 있습니다.
 compress.selectText.2=최적화 레벨:
@@ -1384,4 +1385,3 @@ validateSignature.cert.version=버전
 validateSignature.cert.keyUsage=키 용도
 validateSignature.cert.selfSigned=자체 서명
 validateSignature.cert.bits=비트
-compress.grayscale.label=압축을 위해 그레이스케일 적용