Downgrade to swift-tools-version 5.9

Drop swift-tools-version from 6.0 to 5.9 for broader compatibility.
Remove swiftLanguageMode(.v6) setting as it is not available in tools 5.9.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: vkuttyp

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 6.0
+// swift-tools-version: 5.9
 import PackageDescription
 
 // SQLite is provided by the Apple SDK on Darwin; on Linux we need a system library.
@@ -154,6 +154,4 @@ let package = Package(
     ] + sqliteSystemLibTargets
 )
 
-var swiftSettings: [SwiftSetting] { [
-    .swiftLanguageMode(.v6),
-] }
+var swiftSettings: [SwiftSetting] { [] }