[Iceberg] Rename "target_split_size" to "target_split_size_bytes"

[ZacBlanco]

Description

Renames the target split size property to make it clear that only a bytes value is accepted.

Motivation and Context

Less confusion

Impact

Renames a session property. This has not been in any release yet, so no serious impact. The original PR has had the release note updated with the correct name

Test Plan

N/A

Contributor checklist

• Please make sure your submission complies with our contributing guide, in particular code style and commit standards.
• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• If release notes are required, they follow the release notes guidelines.
• Adequate tests were added if applicable.
• CI passed.

Release Notes

== NO RELEASE NOTE ==

[steveburnett]

LGTM! (docs)

Pull branch, local doc build, looks good. Thanks!