Bump ch.qos.logback:logback-classic from 1.5.19 to 1.5.21 (closes #167)

Bumps [ch.qos.logback:logback-classic](https://github.com/qos-ch/logback) from 1.5.19 to 1.5.21.
- [Release notes](https://github.com/qos-ch/logback/releases)
- [Commits](https://github.com/qos-ch/logback/compare/v_1.5.19...v_1.5.21)

Change-Id: Ia015a38560bee560a52eb96c060cc2160a907c73

---
updated-dependencies:
- dependency-name: ch.qos.logback:logback-classic
  dependency-version: 1.5.21
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Change-Id: I56997add3071a37af0f934eceba24caefd8e15d3
diff --git a/pom.xml b/pom.xml
index f2459c2..595c3ab 100644
--- a/pom.xml
+++ b/pom.xml
@@ -118,7 +118,7 @@
     <dependency>
       <groupId>ch.qos.logback</groupId>
       <artifactId>logback-classic</artifactId>
-      <version>1.5.19</version>
+      <version>1.5.21</version>
       <scope>test</scope>
     </dependency>
     <dependency>