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_zh_TW.properties

@@ -951,6 +951,7 @@ fileToPDF.submit=轉換為 PDF
 compress.title=壓縮
 compress.header=壓縮 PDF
 compress.credit=此服務使用 qpdf 進行 PDF 壓縮/最佳化。
+compress.grayscale.label=應用灰階進行壓縮
 compress.selectText.1=手動模式 - 從 1 到 5
 compress.selectText.1.1=在最佳化等級 6 到 9 時，除了一般 PDF 壓縮外，圖片解析度也會降低以進一步減少檔案大小。較高的壓縮等級會進行更高強度的圖片壓縮（最多可壓縮到原始大小的 50%），以達到更高的壓縮率，但可能會影響圖片品質。
 compress.selectText.2=最佳化等級：
@@ -1384,4 +1385,3 @@ validateSignature.cert.version=版本
 validateSignature.cert.keyUsage=金鑰用途
 validateSignature.cert.selfSigned=自我簽署
 validateSignature.cert.bits=位元
-compress.grayscale.label=應用灰階進行壓縮