Missing S3 support for eu-central-2

[Ahsan Zaheer]

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Environment

• Elixir & Erlang versions (elixir --version):

Erlang/OTP 25 [erts-13.2.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] [dtrace]

Elixir 1.14.3 (compiled with Erlang/OTP 25)

• ExAws version mix deps |grep ex_aws

* ex_aws 2.5.0 (Hex package) (mix)
  locked at 2.5.0 (ex_aws) 971b86e5
* ex_aws_s3 2.4.0 (Hex package) (mix)
  locked at 2.4.0 (ex_aws_s3) 85dda6e2

• HTTP client version. IE for hackney do mix deps | grep hackney

* hackney 1.18.1 (Hex package) (rebar3)
  locked at 1.18.1 (hackney) a4ecdaff

Current behavior

    ** (RuntimeError) s3 not supported in region eu-central-2 for partition aws
        (ex_aws 2.5.0) lib/ex_aws/config/defaults.ex:180: ExAws.Config.Defaults.fetch_or/3
        (ex_aws 2.5.0) lib/ex_aws/config/defaults.ex:163: ExAws.Config.Defaults.do_host/3
        (ex_aws 2.5.0) lib/ex_aws/config/defaults.ex:94: ExAws.Config.Defaults.get/2
        (ex_aws 2.5.0) lib/ex_aws/config.ex:96: ExAws.Config.build_base/2
        (ex_aws 2.5.0) lib/ex_aws/config.ex:69: ExAws.Config.new/2
        (ex_aws 2.5.0) lib/ex_aws.ex:73: ExAws.request/2
        ```

Include code samples, errors and stacktraces if appropriate.

### Expected behavior
Well, I've verified S3 works on eu-central-2 through the AWS cli, but not working with `ex_aws`

[Ahsan Zaheer]

Fix: #981