fix(deps): update module github.com/aws/aws-sdk-go-v2/service/sns to v1.21.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
github.com/aws/aws-sdk-go-v2/service/sns	require	minor	v1.20.15 -> v1.21.0

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Release Notes

aws/aws-sdk-go-v2 (github.com/aws/aws-sdk-go-v2/service/sns)

### [`v1.21.0`](https://togithub.com/aws/aws-sdk-go-v2/compare/service/sts/v1.20.0...service/pinpoint/v1.21.0)

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