[spring] fix void return type for useResponseEntity: false

[martin-mfg]

fix #15966

For some configurations, illegal code like Mono<void> is created, which should be Mono<Void> (with uppercase "V") instead. This PR fixes the problem.

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[wing328]

@martin-mfg Thanks for the fix. What about adding a test in modules/openapi-generator/src/test/resources/3_0/spring/petstore-with-fake-endpoints-models-for-testing.yaml ?

[martin-mfg]

@wing328 I added a new test.

The bug is triggered if reactive: true and useResponseEntity: false. So I created a new sample config with these settings, as none of the existing sample configs had this combination. modules/openapi-generator/src/test/resources/3_0/spring/petstore-with-fake-endpoints-models-for-testing.yam didn't even need to be changed, it contained already endpoints for which the bug was triggered.