[Bug]: Bedrock client uses incorrect environment variables for authentication

[ghost]

Why are these changes needed?

The AWS Bedrock client uses AWS_ACCESS_KEY and AWS_SECRET_KEY as environment variables., the names of which has been updated. in ref: here these changes are to update the code base to keep in line with the same.

Related issue number

Closes #3500

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[ghost]

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[ekzhu]

LGTM