Fix :checksum_mode param in S3::FileDownloader

[jterapin]

#3317

Context

When we introduced new request/response checksum configuration options, checksum validation was determined at the client config level. However, documentation on FileDownloader implies that setting :checksum_mode to "DISABLED" will disable the response checksum validation.

The Problem

• Even when :checksum_mode was set to "DISABLED", checksums were still being calculated silently due to the client's config setting for :response_checksum_validation

• Recent FileDownloader optimizations added parameter validation that now incorrectly passes the deprecated checksum_mode: "DISABLED" to HEAD/GET object operations, causing errors ("DISABLED" is not a valid value)

Proposed solution

In response, this PR addresses the following:

• Deprecates the :checksum_mode parameter in favor of the client's :response_checksum_validation configuration
• When :checksum_mode is set to "DISABLED", shows a deprecation warning and skips the parameter to prevent errors
• Updates documentation for clarity on proper checksum configuration

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