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_es_ES.properties

@@ -951,6 +951,7 @@ fileToPDF.submit=Convertir a PDF
 compress.title=Comprimir
 compress.header=Comprimir PDF
 compress.credit=Este servicio utiliza qpdf para compresión/optimización de PDF
+compress.grayscale.label=Aplicar escala de grises para compresión
 compress.selectText.1=Modo manual - De 1 a 5
 compress.selectText.1.1=En los niveles de optimización 6 a 9, además de la compresión general de PDF, se reduce la resolución de la imagen para reducir aún más el tamaño del archivo. Los niveles más altos dan como resultado una mayor compresión de la imagen (hasta el 50 % del tamaño original), lo que permite lograr una mayor reducción del tamaño, pero con una posible pérdida de calidad en las imágenes.
 compress.selectText.2=Nivel de optimización:
@@ -1384,4 +1385,3 @@ validateSignature.cert.version=Versión
 validateSignature.cert.keyUsage=Uso de la llave
 validateSignature.cert.selfSigned=Autofirmado
 validateSignature.cert.bits=bits
-compress.grayscale.label=Aplicar escala de grises para compresión