Update default Magento version to 2.4.6-p3.

[cosmicvibes]

No description provided.

[what-the-diff]

PR Summary

• Version Update Across Multiple Files
  The version of the product has been upgraded from 2.4.6-p2 to 2.4.6-p3. This change affects several files including README.md, compose/bin/download, and lib/onelinesetup.

• Default Version and Type Modification in 'bin/download'
  In the README.md file, guidance has been updated, specifying that the default values for the version (now 2.4.6-p3) and type (now community) used in the bin/download command have been adjusted.

• VERSION Variable Modification in compose/bin/download and lib/onelinesetup
  The VERSION variable present in both compose/bin/download and lib/onelinesetup files has also been updated from 2.4.6-p2 to 2.4.6-p3 to maintain version consistency across the application.

[markshust]

Thanks so much for this PR! Looks great, merging it in 👍