refactor: remove deprecated .upper() and .lower() methods

Removes backward-compatible .upper() and .lower() methods in favor of
the more Swifty .uppercased() and .lowercased() API.

## Changes

- Removed QueryExpression extension with .upper() and .lower()
- Updated tests to use .uppercased() and .lowercased()
- All 860 tests passing

## API Migration

- .upper() → .uppercased()
- .lower() → .lowercased()

Users should use the Swifty API which matches Swift.String conventions.

Author: coenttb

Sources/StructuredQueriesPostgres/Functions/String/String+CaseConversion.swift

@@ -97,36 +97,6 @@ extension PostgreSQL.String {
 
 // MARK: - QueryExpression Extension (Fluent API)
 
-extension QueryExpression where QueryValue: _OptionalPromotable<Swift.String?> {
-    /// Converts the string to lowercase
-    ///
-    /// PostgreSQL's `lower()` function.
-    ///
-    /// ```swift
-    /// User.select { $0.email.lower() }
-    /// // SELECT lower("users"."email") FROM "users"
-    /// ```
-    ///
-    /// - Returns: An expression wrapped with the `lower` function.
-    public func lower() -> some QueryExpression<QueryValue> {
-        QueryFunction("lower", self)
-    }
-
-    /// Converts the string to uppercase
-    ///
-    /// PostgreSQL's `upper()` function.
-    ///
-    /// ```swift
-    /// User.select { $0.name.upper() }
-    /// // SELECT upper("users"."name") FROM "users"
-    /// ```
-    ///
-    /// - Returns: An expression wrapped with the `upper` function.
-    public func upper() -> some QueryExpression<QueryValue> {
-        QueryFunction("upper", self)
-    }
-}
-
 extension QueryExpression where QueryValue == Swift.String {
     /// Converts the string to uppercase
     ///

Tests/StructuredQueriesPostgresTests/Functions/String/StringFunctionsTests.swift

@@ -24,9 +24,9 @@ extension SnapshotTests.StringFunctions {
 
         // MARK: - Case Conversion Tests
 
-        @Test func upper() async {
+        @Test func uppercased() async {
             await assertSQL(
-                of: Person.select { $0.name.upper() }
+                of: Person.select { $0.name.uppercased() }
             ) {
                 """
                 SELECT upper("persons"."name")
@@ -35,9 +35,9 @@ extension SnapshotTests.StringFunctions {
             }
         }
 
-        @Test func lower() async {
+        @Test func lowercased() async {
             await assertSQL(
-                of: Person.select { $0.email.lower() }
+                of: Person.select { $0.email.lowercased() }
             ) {
                 """
                 SELECT lower("persons"."email")