Apply item attribute key fix to base_v1_21_4

[JToTl]

This is a follow-up to #1506. The fix in commit 958fcb0 added the key-aware setItemAttribute overload and updated the v26 implementations, but base_v1_21_4 was not updated.

base_v1_21_4 continued to inherit setItemAttribute from base_v1_20_5. The inherited implementation does not use attributeKey; it calls createAttributeModifier with the UUID instead. The existing override in base_v1_21_4 then converts this to the constant key magic:modifier, so multiple modifiers can still receive the same key.

This change overrides the key-aware overload in base_v1_21_4 and creates the modifier using the supplied key. When no key is specified, it generates the same default key used by the v26 implementations: modifier_<attribute>_<slot>.

The five- and six-argument overloads remain inherited from base_v1_20_5 and delegate to this override.