out_splunk: Extend retrying HTTP status code

[cosmo0920]

408 and 429 also should be treated as retry.

Related to #9371.

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[meehanman]

This should be 429 and not 428.

429

You've made too many requests

Splunk Secure Gateway cannot handle that many requests at a time. Lower your number of requests and try again.

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Additionally I'll try and ping some of the Splunk contributors for their opinion.

[cosmo0920]

This should be 429 and not 428.

429

You've made too many requests

Splunk Secure Gateway cannot handle that many requests at a time. Lower your number of requests and try again.

Oh, sorry. I made a mistake. I fixed it.