bump ch.qos.logback.version from 1.5.5 to 1.5.6

---
updated-dependencies:
- dependency-name: ch.qos.logback:logback-classic
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: ch.qos.logback:logback-core
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -96,7 +96,7 @@
     <com.io7m.zelador.version>0.0.1</com.io7m.zelador.version>
 
     <!-- Third-party dependencies. -->
-    <ch.qos.logback.version>1.5.5</ch.qos.logback.version>
+    <ch.qos.logback.version>1.5.6</ch.qos.logback.version>
     <com.github.davidmoten.subethasmtp.version>7.0.2</com.github.davidmoten.subethasmtp.version>
     <com.helger.ph-css.version>7.0.2</com.helger.ph-css.version>
     <com.squareup.okio.version>3.9.0</com.squareup.okio.version>