Invalid AWS Mock Access Key ID through a new AWS-SDK version

Due to a new AWS_SDK version only the following ACCESS KEY ID and SECRETS are allowed:

` DynamoDB local version 2.0.0 and greater AWS_ACCESS_KEY_ID can contain the only letters (A–Z, a–z) and numbers (0–9).`

[Stackoverflow thread](https://stackoverflow.com/questions/76592141/aws-js-sdk-suddenly-throws-the-access-key-id-or-security-token-is-invalid)

I've already created a PR to fix this issue:
https://github.com/99x/serverless-dynamodb-local/pull/298

Review pull request #298 first, since the issue says the fix is already proposed. Verify the change against the stated AWS_ACCESS_KEY_ID restrictions and confirm the local DynamoDB setup works with the newer AWS SDK version.

Written by the indexing model from the issue text.