Remove inaccurate s3 mentions from the code

[arpad-m]

Related to #5567, I wouldn't say it's a hard blocker though as it only affects internal parts.

There is log messages/error messages in the code of the pageserver, s3 scrubber etc that mention S3. But with azure blob storage present (added by #5546), these messages are not accurate any more: they can also be about S3. We should find an alternative that is not too long, but more accurately reflects that we interact with some remote storage, whether it's S3, azure blobs, or the local file system. Then we should migrate the log lines to that alternative.

Obviously, code that is still specific to S3, e.g. due to directly interacting with the AWS SDK instead of going through the remote storage abstraction can still mention S3. This is only about the ones where the use is not accurate.