Update module github.com/DataDog/zstd to v1.5.6

Open renovate requested to merge renovate/github.com-datadog-zstd-1.x into master 6 months ago

This MR contains the following updates:

Package	Type	Update	Change
github.com/DataDog/zstd	require	patch	`v1.5.5` -> `v1.5.6`

DataDog/zstd (github.com/DataDog/zstd)

Fix readme typo by @colinlyguo in https://github.com/DataDog/zstd/pull/136
Update upperBound ratio when guessing the required decompression buffer size by @sfluor in https://github.com/DataDog/zstd/pull/141
Update vendored zstd to 1.5.6 by @Viq111 in https://github.com/DataDog/zstd/pull/143

Full Changelog: https://github.com/DataDog/zstd/compare/v1.5.5+patch1...v1.5.6