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_sl_SI.properties

@@ -951,6 +951,7 @@ fileToPDF.submit=Pretvori v PDF
 compress.title=Stisnite
 compress.header=Stisnite PDF
 compress.credit=Ta storitev uporablja qpdf za stiskanje/optimizacijo PDF.
+compress.grayscale.label=Uporabi sivinsko lestvico za stiskanje
 compress.selectText.1=Ročni način - Od 1 do 5
 compress.selectText.1.1=Na stopnjah optimizacije od 6 do 9 je poleg splošnega stiskanja PDF ločljivost slike zmanjšana, da se dodatno zmanjša velikost datoteke. Višje ravni povzročijo močnejše stiskanje slike (do 50 % prvotne velikosti), s čimer se doseže večje zmanjšanje velikosti, vendar s potencialno izgubo kakovosti slik.
 compress.selectText.2=Raven optimizacije:
@@ -1384,4 +1385,3 @@ validateSignature.cert.version=Različica
 validateSignature.cert.keyUsage=Uporaba ključa
 validateSignature.cert.selfSigned=Samopodpisano
 validateSignature.cert.bits=bits
-compress.grayscale.label=Uporabi sivinsko lestvico za stiskanje