Finalize Variant and shredding specs (#509)

Author: aihuaxu

VariantEncoding.md

@@ -19,9 +19,6 @@
 
 # Variant Binary Encoding
 
-> [!IMPORTANT]
-> **This specification is still under active development, and has not been formally adopted.**
-
 A Variant represents a type that contains one of:
 - Primitive: A type and corresponding value (e.g. INT, STRING)
 - Array: An ordered list of Variant values

VariantShredding.md

@@ -19,9 +19,6 @@
 
 # Variant Shredding
 
-> [!IMPORTANT]
-> **This specification is still under active development, and has not been formally adopted.**
-
 The Variant type is designed to store and process semi-structured data efficiently, even with heterogeneous values.
 Query engines encode each Variant value in a self-describing format, and store it as a group containing `value` and `metadata` binary fields in Parquet.
 Since data is often partially homogeneous, it can be beneficial to extract certain fields into separate Parquet columns to further improve performance.