fix: using Micronaut's Response.getReason instead of javax core Response

[jmktam]

Using Micronaut's Response.getReason instead of javax core Response.Status.getReasonPhrase.

Issue:
When using AWS ALB, the ResponseWriter attempts to get get the response phrase from javax core's Response.Status. However, javax core's Respons.Status implements a subset of Http Response codes that Micronaut supports. This results in a null pointer exception.

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