deps: update logback.version to v1.2.6 (#528)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [ch.qos.logback:logback-core](http://logback.qos.ch) ([source](https://togithub.com/ceki/logback)) | `1.2.5` -> `1.2.6` | [![age](https://badges.renovateapi.com/packages/maven/ch.qos.logback:logback-core/1.2.6/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/ch.qos.logback:logback-core/1.2.6/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/ch.qos.logback:logback-core/1.2.6/compatibility-slim/1.2.5)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/ch.qos.logback:logback-core/1.2.6/confidence-slim/1.2.5)](https://docs.renovatebot.com/merge-confidence/) |
| [ch.qos.logback:logback-classic](http://logback.qos.ch) ([source](https://togithub.com/ceki/logback)) | `1.2.5` -> `1.2.6` | [![age](https://badges.renovateapi.com/packages/maven/ch.qos.logback:logback-classic/1.2.6/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/ch.qos.logback:logback-classic/1.2.6/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/ch.qos.logback:logback-classic/1.2.6/compatibility-slim/1.2.5)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/ch.qos.logback:logback-classic/1.2.6/confidence-slim/1.2.5)](https://docs.renovatebot.com/merge-confidence/) |

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pom.xml

@@ -14,7 +14,7 @@
   </description>
 
   <properties>
-    <logback.version>1.2.5</logback.version>
+    <logback.version>1.2.6</logback.version>
     <easymock.version>3.6</easymock.version>
     <truth.version>1.1.3</truth.version>
     <logging.version>3.1.1</logging.version>