Switch to AWS SDK v2 before v1 is end-of-support/EOL on July 31, 2025 - CT

According to this https://aws.amazon.com/blogs/developer/announcing-end-of-support-for-aws-sdk-for-go-v1-on-july-31-2025/, there will be no more releases for AWS SDK for Go v1 after July 2025. AWS as maintainer of this AWS SDK for Go package announced the **EOL of the version 1 for July 31, 2025**. After this date, there are **no** critical bug fixes and **no** security issues provided. Furthermore, the v1 SDK will not receive API updates for new or existing services, or be updated to support new regions.

Example for require-reference (it's indirect):
https://github.com/google/certificate-transparency-go/blob/dd49d3b91e8fada33ccb79bbfd22736917e4e311/go.mod#L49

CT team should consider identifying and updating modules/deps, which still using `"aws-sdk-go-v1"`. Once done, an update of this module and a new release should be provided (to enable also other projects, which using this project here). Such migration to [AWS SDK for Go v2](https://aws.github.io/aws-sdk-go-v2/docs/) - [`aws-sdk-go-v2`](https://github.com/aws/aws-sdk-go-v2/) should reduce the technical dept and converges to use a single version of this SDK. There is a migration guide available: https://aws.github.io/aws-sdk-go-v2/docs/migrating/

Start with go.mod and identify modules and dependencies that still reference aws-sdk-go-v1, then read the AWS SDK for Go v2 migration guide. Done means updating the affected dependencies, completing the migration, and publishing a new release of certificate-transparency-go.

Written by the indexing model from the issue text.