Rely on org.springframework.core.Nullness to determine if an Actuator endpoint is optional

[mhalbritter]

In org.springframework.boot.actuate.endpoint.invoke.reflect.OperationMethodParameter, we check for org.springframework.lang.Nullable and for javax.annotation.Nonnull. We should also add support JSpecify.

[snicoll]

Closing in favor of PR #46595

[snicoll]

Rather than doing a check on JSpecify ourselves, we should rely on org.springframework.core.Nullness that abstracts this for us and provide a consistent experience with the rest of the codebase.