[java][okhttp] Bump OkHTTP dependency to latest

[toby-murray-snow-software]

In 5.0.1 the generated line in ApiClient was:

return RequestBody.create(MediaType.parse(contentType), (byte[]) obj);

in 5.1.0 it is:

return RequestBody.create((byte[]) obj, MediaType.parse(contentType));

Looks like this change was introduced in #8969, and requires a more recent version of OkHttp to compile in some (all?) circumstances.

I tested out the failing spec that was mentioned in the linked ticket, and it compiles now.

To fix #9151

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[wing328]

Tested locally and the result is good.