Update liquid gem dependency when liquid#1772 is fixed

[ronaldtse]

As per @suleman-uzair :

TODO: Remove once this is fixed:

• Ruby 3.3 deprecation warnings: base64, bigdecimal will no longer be part of the default gems Shopify/liquid#1772

Originally posted by @ronaldtse in #369 (comment)

[ronaldtse]

Closing: our dependency admits liquid 4.x through 5.x, current releases (5.14.x) work, and the upstream base64/bigdecimal deprecation tracking happens in Shopify's own issue — nothing actionable remains here. If a future liquid release requires a dependency or code change on our side, a fresh task will be opened at that time.