Bump com.diffplug.spotless:spotless-maven-plugin in the maven group

Bump com.diffplug.spotless:spotless-maven-plugin in the maven group

Bumps the maven group with 1 update: [com.diffplug.spotless:spotless-maven-plugin](https://github.com/diffplug/plugin-maven).

Updates `com.diffplug.spotless:spotless-maven-plugin` from 1.18.0 to 1.20.0
- [Commits](https://github.com/diffplug/plugin-maven/commits)

updated-dependencies:
- dependency-name: com.diffplug.spotless:spotless-maven-plugin
  dependency-type: direct:production
  dependency-group: maven

Issue-ID: CIMAN-33

Change-Id: Ieaf1b99a11bd5e1eff5c1ed1a906f56ac438e998
Signed-off-by: dependabot[bot] <support@github.com>
Signed-off-by: onap.gh2gerrit <releng+onap-gh2gerrit@linuxfoundation.org>