Update cache S3 SSE Key ID docs

What does this MR do?

This MR updates the docs for GitLab Runner's Cache S3 setting related to server side encryption key id.

Why was this MR needed?

When the runner runs in AWS Account 1 and its cache is stored in AWS Account 2, specifying the SSE ID/alias results in searching for the key in AWS Account 1 instead of AWS Account 2 where it actually exists. This can be achieved by specifying the ARN of the key which has the account information.

What's the best way to test this MR?

Looking at our code, I found out that

• The key is read from the environment variable here in the CacheS3Config struct.
• The tests for this file already specifies this value as an ARN here.
• This key is used in the cache.s3v2.s3Adapter.GetGoCloudURL function here. The upstream library also mentions this key as an ARN in their tests.

We also have a confirmation from an end user who faced this issue - #37879 (comment 2840311072)

What are the relevant issue numbers?

• #37879 (comment 2840311072)