Require explicit setting of [runners.cache.s3.AuthenticationType]

In !3117 (merged), we added a configuration option in [runners.cache.s3] called AuthenticationType

This allows four configuration paths. Two of which we want to deprecate in %15.0.

Deprecated configuration options

1. No AuthenticationType is set, no ServerAddress, AccessKey or SecretKey - Runner assumes that it should use IAM. This should be marked as deprecated and tagged for removal in %15.0.

2. No AuthenticationType is set, AccessKey and SecretKey, and ServerAddress are all assigned. - Runner uses explicit credentials and nothing else is needed. This should be also marked as deprecated and tagged for removal in %15.0.'

New configuration options

1. AuthenticationType set to IAM - Runner will use the IAM authentication strategy.

2. AuthenticationType set to access-key - Runner will use AWS root credentials.

If AuthenticationType is not set, we need to decide how we want the default behavior to be. Either default to AWS root credentials, or IAM.