deps(build): centralize Logback version management and update to 1.5.21 (#4868)

# Description of Changes

- Introduced a new Gradle property `logback` in the `ext` block for
centralized version management.
- Updated Logback dependencies (`logback-core`, `logback-classic`) from
version `1.5.20` to `1.5.21`.
- Replaced hardcoded versions in subprojects with dynamic references to
the new `logback` variable.

This improves maintainability and ensures consistent versioning across
all modules.

---

## 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/devGuide/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/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] 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/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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/devGuide/DeveloperGuide.md#6-testing)
for more details.

build.gradle

@@ -30,6 +30,7 @@ ext {
     openSamlVersion = "4.3.2"
     commonmarkVersion = "0.27.0"
     googleJavaFormatVersion = "1.28.0"
+    logback = "1.5.21"
     tempJrePath = null
 }
 
@@ -126,8 +127,8 @@ subprojects {
         implementation 'io.github.pixee:java-security-toolkit:1.2.2'
 
         //tmp for security bumps
-        implementation 'ch.qos.logback:logback-core:1.5.20'
-        implementation 'ch.qos.logback:logback-classic:1.5.20'
+        implementation "ch.qos.logback:logback-core:$logback"
+        implementation "ch.qos.logback:logback-classic:$logback"
         compileOnly "org.projectlombok:lombok:$lombokVersion"
         annotationProcessor "org.projectlombok:lombok:$lombokVersion"