HHH-15557 fix problem when orm plugin is applied before Java plugin

[der-ambi]

There was a report that the build.gradle file contained in the project stubs generated by start.spring.io did not compile (see spring-io/start.spring.io#1078) that identified HHH-15557 as the underlying issue.

Here is my attempt to fix it following the approach suggested in the Gradle docs.

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