Change existing AWS service connection authentication from access & secret key to OIDC

**Is your feature request related to a problem? Please describe.**
I tried to modify an existing AWS service connection to switch from access & secret key authentication to OIDC authentication. Using the service connection editor I cleared the access key and checked the "Use OIDC" checkbox (note that I could do nothing with the secret key because it is already empty according to the UI). I then re-ran a pipeline that was using the service connection and received this error:

"##[error]Error: Need to define or omit both "Access Key ID" and "Secret Access Key", not just one."

I strongly suspect this is because I was only able to clear the access key field while the secret key remained unchanged.

**Describe the solution you'd like**
It would be very helpful if the service connection UI had some technique for clearing the secret key or if the runtime just logged a message that OIDC authentication is being used because incomplete access key/secret key credentials were configured in the service connection.

**Describe alternatives you've considered**
I considered asking customers to create a new service connection that uses OIDC but then they also have to modify all of their pipelines to use the new service connection. It would be much easier if I could provide instructions to them on how to modify their existing service connections.

**Additional context**
Organizationally, I work for group that sets standards for our whole enterprise so I am looking for the most efficient way to convert our whole enterprise to use OIDC for AWS service connections.

Start with the service connection editor and the pipeline runtime path described in the report. Reproduce conversion of an existing AWS connection by clearing the access key and enabling OIDC, then rerun the pipeline; done means the existing connection uses OIDC without requiring a replacement or reporting incomplete key credentials.

Written by the indexing model from the issue text.