Add missing AWS Regions for validation

[ewbankkit]

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Relates hashicorp/terraform-provider-aws#23116.
Adds missing ap-northeast-3 and ap-southeast-3 AWS Regions (plus others) for validation.

% go test -v '-run=TestValidateRegion' .
=== RUN   TestValidateRegion
=== RUN   TestValidateRegion/us-east-1
=== RUN   TestValidateRegion/ap-northeast-3
=== RUN   TestValidateRegion/us-gov-west-1
=== RUN   TestValidateRegion/cn-northwest-1
=== RUN   TestValidateRegion/invalid
--- PASS: TestValidateRegion (0.00s)
    --- PASS: TestValidateRegion/us-east-1 (0.00s)
    --- PASS: TestValidateRegion/ap-northeast-3 (0.00s)
    --- PASS: TestValidateRegion/us-gov-west-1 (0.00s)
    --- PASS: TestValidateRegion/cn-northwest-1 (0.00s)
    --- PASS: TestValidateRegion/invalid (0.00s)
PASS
ok  	github.com/hashicorp/aws-sdk-go-base/v2	0.242s

Previously

=== RUN   TestValidateRegion/ap-northeast-3
    validation_test.go:40: Expected no error, received error: Invalid AWS Region: ap-northeast-3

[YakDriver]

Looks great! 🎉

[gdavison]

Re-merged to the main branch