fix: remove AWS SDK v1 dependencies and use AWS SDK v2 in neptune backend

[gaganhr94]

Description of the PR

Replace github.com/aws/aws-sdk-go (v1) with github.com/aws/aws-sdk-go-v2 in the Neptune backend. The v1 SDK is deprecated and no longer receiving updates. The v2 equivalents for session, credentials, and signer/v4 are used, preserving the existing credential resolution logic (env vars first, then default config fallback)

Fixes #2912

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If GraphQL schema is changed, GraphQL client updates/additions have been made
• If OpenAPI spec is changed, make generate has been run
• If ent schema is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[mihaimaruseac]

Thank you!

[mihaimaruseac]

We also need to resolve conflicts here

[gaganhr94]

Done @mihaimaruseac, it is rebased with main now

[mihaimaruseac]

Awesome. We need one more reviewer and then it should merge