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

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 (there might be more):
https://github.com/grafana/alerting/blob/1d263576d356324caee7ea20fcb5aee3df5e165b/go.mod#L11

Grafana Alerting Library should consider migrating 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/) so that we 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 at the referenced dependency and search the repository for other AWS SDK for Go v1 references. Read the AWS SDK for Go v2 migration guide before mapping the required changes. Done means the Grafana Alerting Library uses AWS SDK for Go v2 consistently instead of the v1 dependency.

Written by the indexing model from the issue text.