Replace all occurrences of zstd_TARGET with zstd::libzstd

[coderabbitai]

Description

A TODO was identified in PR #1479 to replace all occurrences of the zstd_TARGET variable with direct usage of the zstd::libzstd target.

Context

In PR #1479, the CMake configuration was updated to always set zstd_TARGET to zstd::libzstd (removing the conditional logic for static vs shared). As a follow-up, all usages of the zstd_TARGET variable should be replaced with direct references to zstd::libzstd to simplify the codebase.

References

• PR: refactor(deps): Document standard rules for writing lib install tasks and apply across all existing tasks: #1479
• Comment: refactor(deps): Document standard rules for writing lib install tasks and apply across all existing tasks: #1479 (comment)
• Requested by: @LinZhihao-723

[Bill-hbrhbr]

Resolved in #2037