False warnings when building core-java/server test data

[alexander-yevsyukov]

Here's what I got recently in the console:

* What went wrong:
Execution failed for task ':server:generateTestProto'.
> protoc: stdout: . stderr: Sep 17, 2022 6:14:58 PM io.spine.validate.option.Required checkUsage
  WARNING: The field `spine.test.aggregate.number.RejectNegativeLong.number` has the type INT64 and should not be declared as `(required)`.
  Sep 17, 2022 6:14:58 PM io.spine.validate.option.Required checkUsage
  WARNING: The field `spine.test.server.number.NaturalNumber.value` has the type INT32 and should not be declared as `(required)`.
...

Looking at the NaturalNumber declaration reveals that it's not so:

message NaturalNumber {
    option (entity).kind = ENTITY;

    // Natural number value.
    //
    // Must be greater than or equal to zero.
    int32 value = 1 [(min).value = "0"];
}

We do check that at least one field should be in an entity state type, but it doesn't make it required.