deps: bump net.logstash.logback:logstash-logback-encoder

Bumps [net.logstash.logback:logstash-logback-encoder](https://github.com/logfellow/logstash-logback-encoder) from 7.0.1 to 7.4.
- [Release notes](https://github.com/logfellow/logstash-logback-encoder/releases)
- [Commits](logfellow/logstash-logback-encoder@logstash-logback-encoder-7.0.1...logstash-logback-encoder-7.4)

---
updated-dependencies:
- dependency-name: net.logstash.logback:logstash-logback-encoder
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

spring-cloud-gcp-logging/pom.xml

@@ -9,7 +9,7 @@
 	</parent>
 
 	<properties>
-		<logstash.version>7.0.1</logstash.version>
+		<logstash.version>7.4</logstash.version>
 	</properties>
 
 	<artifactId>spring-cloud-gcp-logging</artifactId>