Config quarkus-extension-processor to use legacy @ConfigRoot

[mcruzdev]

Many thanks for submitting your Pull Request ❤️!

I created an issue #793, to use @ConfigMapping. This pull request just solves the build.

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[hbelmiro]

@mcruzdev can we add the arg only to the default-compile execution to avoid warnings in tests?
See the following section from https://github.com/quarkusio/quarkus/wiki/Migration-Guide-3.14#for-extension-developers for more context:

Note that this will result in a warning when compiling the test classes (given you don’t have config annotations in your test classes, the annotation processor will not be triggered and you will get a warning telling you the legacyConfigRoot parameter is unused). To alleviate this issue, it is recommended to only enable the annotation processor for the default-compile execution (which compiles the main classes):

             <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <annotationProcessorPaths>
                        <path>
                            <groupId>io.quarkus</groupId>
                            <artifactId>quarkus-extension-processor</artifactId>
                            <version>${quarkus.version}</version>
                        </path>
                    </annotationProcessorPaths>
                    <compilerArgs>
                        <arg>-AlegacyConfigRoot=true</arg>
                    </compilerArgs>
                </configuration>
            </plugin>

Also, we need to migrate to the new @ConfigMapping approach in a follow-up PR. Is #793 intended for that? Otherwise we need a new issue.

[ricardozanini]

I agree to migrate to a more final solution rather than the legacy workaround.