Previously, the mavenOptional was added to every published module but it
was only used by spring-boot-maven-plugin. This commit reduces its scope
so that it only affects the Maven plugin. It also reworks the
implementation to reuse the existing optional configuration rather than
declaring a new mavenOptional configuration. Lastly, publication of
Gradle Module Metadata (GMM) has been disabled for
spring-boot-maven-plugin. This is seen as preferable to publishing the
metadata – which isn't really needed as it does not contain any useful
additional information – and having to suppress warnings about
incomplete mapping of GMM to pom metadata.
Closes gh-41263
365fdfee45