chore: Remove warning about using long-term credentials

[tim-finnigan]

Issue #, if available: #885

Description of changes: #871 added a warning about using long-term credentials. Upon discussion with the team, we think that this should be called out in the README rather than in a warning. And there is already a recommendation in the README to use OIDC to get short-lived credentials:

We recommend using GitHub's OIDC provider to get short-lived AWS credentials needed for your actions.

Although long-term credentials may not be recommended in most cases, the AWS IAM documentation does note:

There are specific use cases that require long-term credentials with IAM users in AWS.

Ultimately Identity and Access Management is the customer's responsibility in the Shared Responsibility Model. We can recommend best practices but should be cautious about adding warnings that may not be universally applicable.

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[jplock]

Can we ask the IAM team if they’ll ever change those prefixes instead of reverting this warning?

The largest attack vector we see within AWS Startups is accounts being compromised due to mishandled access keys. We actively want to discourage their usage and GitHub Actions is a source of key exposure.

Before the short link was removed in #905, there were 741 clicks on the link to read about OIDC.

[haooliveira84]

@kellertk when this PR will be part of v4?