Use sdk v3 now that fluent-plugin-s3 also uses it

[grosser]

removes a ton of dependencies when installing https://rubygems.org/gems/fluent-plugin-s3

@riywo @adammw

⚠️note-to-self do not force-push since we use that commit ⚠️

[simukappu]

Hi @grosser,
Thank you for your contribution! Now we are discussing that we're going to merge #156 including your #152. Then, we will release these update as v3.0.0-rc (#158).

Before merging your contribution, could you confirm to the formal statement below?:

"Please confirm this contribution is under the terms of the Apache 2.0 license. Thanks."

[grosser]

I confirm this contribution is under the terms of the Apache 2.0 license.

[simukappu]

Thank you - @grosser! We will merge this as soon as possible.