Do not ask customers to create long term credentials for IAM users to use in your demo

It is not cool to ask our customers to use long-term access keys with IAM users in guidance. even example guidance.

https://aws.github.io/bedrock-agentcore-starter-toolkit/user-guide/identity/quickstart.html

Start with the identity quickstart at https://aws.github.io/bedrock-agentcore-starter-toolkit/user-guide/identity/quickstart.html and locate the guidance asking customers to create long-term IAM user credentials. Update the guidance so the demo no longer asks for long-term credentials; done means the quickstart's credential instructions use an appropriate alternative.

Written by the indexing model from the issue text.