Address warnings generated by Maven plugin when used with Maven 3.9

[snicoll]

Maven 3.9 ships additional checks that lead to warnings at the end of the build. Here's the verbose output:

[WARNING]  * io.spring.javaformat:spring-javaformat-maven-plugin:0.0.39
[WARNING]   Declared at location(s):
[WARNING]    * io.spring.initializr:initializr:${revision} (pom.xml) @ line 92
[WARNING]   Used in module(s):
[WARNING]    * io.spring.initializr:initializr:0.20.0-SNAPSHOT (pom.xml)
[WARNING]    * io.spring.initializr:initializr-bom:0.20.0-SNAPSHOT (initializr-bom/pom.xml)
[WARNING]    * io.spring.initializr:initializr-parent:0.20.0-SNAPSHOT (initializr-parent/pom.xml)
[WARNING]    * io.spring.initializr:initializr-generator:0.20.0-SNAPSHOT (initializr-generator/pom.xml)
[WARNING]    * io.spring.initializr:initializr-metadata:0.20.0-SNAPSHOT (initializr-metadata/pom.xml)
[WARNING]    * io.spring.initializr:initializr-generator-test:0.20.0-SNAPSHOT (initializr-generator-test/pom.xml)
[WARNING]    * io.spring.initializr:initializr-generator-spring:0.20.0-SNAPSHOT (initializr-generator-spring/pom.xml)
[WARNING]    * io.spring.initializr:initializr-web:0.20.0-SNAPSHOT (initializr-web/pom.xml)
[WARNING]    * io.spring.initializr:initializr-actuator:0.20.0-SNAPSHOT (initializr-actuator/pom.xml)
[WARNING]    * io.spring.initializr:initializr-docs:0.20.0-SNAPSHOT (initializr-docs/pom.xml)
[WARNING]    * io.spring.initializr:initializr-service-sample:0.20.0-SNAPSHOT (initializr-service-sample/pom.xml)
[WARNING]    * io.spring.initializr:initializr-version-resolver:0.20.0-SNAPSHOT (initializr-version-resolver/pom.xml)
[WARNING]   Plugin issue(s):
[WARNING]    * Plugin should declare these Maven artifacts in `provided` scope: [org.apache.maven:maven-plugin-api:3.5.0, org.apache.maven:maven-model:3.5.0, org.apache.maven:maven-artifact:3.5.0]