Incompatibility with logstash-logback-encoder version 7.1+

[emmileaf]

Opening an issue to track blocker for upgrading logstash-logback-encoder from version 7.0.1, for more visibility to context and past investigations in closed/superseded PRs.

Blocked PRs for upgrade to 7.3: #1600, #1601
Incompatibility from 7.1 bump: #1065 (comment)
Proposed workaround from 7.2 bump: #1138

Error:  Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.10.1:compile (default-compile) on project spring-cloud-gcp-logging: Compilation failure: Compilation failure: 
Error:  /home/runner/work/spring-cloud-gcp/spring-cloud-gcp/spring-cloud-gcp-logging/src/main/java/com/google/cloud/spring/logging/extensions/LogstashLoggingEventEnhancer.java:[45,64] error: getFieldValue() has protected access in ObjectAppendingMarker
Error:  /home/runner/work/spring-cloud-gcp/spring-cloud-gcp/spring-cloud-gcp-logging/src/main/java/com/google/cloud/spring/logging/extensions/LogstashLoggingEventEnhancer.java:[51,78] error: getFieldValue() has protected access in ObjectAppendingMarker

[zhumin8]

Now 7.4 is out.
#1965, #1969