Maven: Use property to select Groovy version in micronaut-groovy-bom

[chrylis]

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The Micronaut BOM (io.micronaut:micronaut-bom) imports org.codehaus.groovy:groovy-bom using the ${groovy.version} property, supporting easy override in end projects. However, io.micronaut.groovy:micronaut-groovy-bom has a duplicate import of the same groovy-bom, but this time it uses an inline version (3.0.3 as of Micronaut 2.0.1). I think that this could be dropped entirely without adverse effect, but if it's retained it should use the same propertied version as the main BOM.