Added compiler checks for Swift 6 specific code

Co-authored-by: Gwendal Roué <gr@pierlis.com>

Author: sroebert

GRDB/Record/FetchableRecord+Decodable.swift

@@ -506,15 +506,19 @@ private struct SingleValueRowDecoder<R: FetchableRecord>: SingleValueDecodingCon
     func decode(_ type: Int16.Type) throws -> Int16 { try columnDecoder.decode(type) }
     func decode(_ type: Int32.Type) throws -> Int32 { try columnDecoder.decode(type) }
     func decode(_ type: Int64.Type) throws -> Int64 { try columnDecoder.decode(type) }
+#if compiler(>=6)
     @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
     func decode(_ type: Int128.Type) throws -> Int128 { try columnDecoder.decode(type) }
+#endif
     func decode(_ type: UInt.Type) throws -> UInt { try columnDecoder.decode(type) }
     func decode(_ type: UInt8.Type) throws -> UInt8 { try columnDecoder.decode(type) }
     func decode(_ type: UInt16.Type) throws -> UInt16 { try columnDecoder.decode(type) }
     func decode(_ type: UInt32.Type) throws -> UInt32 { try columnDecoder.decode(type) }
     func decode(_ type: UInt64.Type) throws -> UInt64 { try columnDecoder.decode(type) }
+#if compiler(>=6)
     @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
     func decode(_ type: UInt128.Type) throws -> UInt128 { try columnDecoder.decode(type) }
+#endif
 
     func decode<T>(_ type: T.Type) throws -> T where T: Decodable {
         if let type = T.self as? any FetchableRecord.Type {