build(deps): update dependency org.apache.maven.plugins:maven-compiler-plugin to v3.9.0 (googleapis#974)

[![WhiteSource Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com) This PR contains the following updates: | Package | Change | Age | Adoption | Passing | Confidence | |---|---|---|---|---|---| | [org.apache.maven.plugins:maven-compiler-plugin](https://maven.apache.org/plugins/) | `3.8.1` -> `3.9.0` | [![age](https://badges.renovateapi.com/packages/maven/org.apache.maven.plugins:maven-compiler-plugin/3.9.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/org.apache.maven.plugins:maven-compiler-plugin/3.9.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/org.apache.maven.plugins:maven-compiler-plugin/3.9.0/compatibility-slim/3.8.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/org.apache.maven.plugins:maven-compiler-plugin/3.9.0/confidence-slim/3.8.1)](https://docs.renovatebot.com/merge-confidence/) | --- ### Configuration 📅 **Schedule**: At any time (no schedule defined). 🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied. ♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox. 🔕 **Ignore**: Close this PR and you won't be reminded about this update again. --- - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, click this checkbox. --- This PR has been generated by [WhiteSource Renovate](https://renovate.whitesourcesoftware.com). View repository job log [here](https://app.renovatebot.com/dashboard#github/googleapis/java-pubsub).

Author: renovate-bot

google-cloud-pubsub/pom.xml

@@ -159,7 +159,7 @@
  <plugins>
  <plugin>
  <artifactId>maven-compiler-plugin</artifactId>
- <version>3.8.1</version>
+ <version>3.9.0</version>
  <configuration>
  <source>1.8</source>
  <target>1.8</target>