Use UnixEpoch for cookie deletion (#18126)

Use the DateTimeOffset.UnixEpoch field instead of a new DateTime value when deleting cookies.

Author: martincostello

src/Http/Http/src/Internal/ResponseCookies.cs

@@ -129,7 +129,7 @@ public void Delete(string key, CookieOptions options)
             {
                 Path = options.Path,
                 Domain = options.Domain,
-                Expires = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc),
+                Expires = DateTimeOffset.UnixEpoch,
                 Secure = options.Secure,
                 HttpOnly = options.HttpOnly,
                 SameSite = options.SameSite

src/Shared/ChunkingCookieManager/ChunkingCookieManager.cs

@@ -288,7 +288,7 @@ public void DeleteCookie(HttpContext context, string key, CookieOptions options)
                     SameSite = options.SameSite,
                     Secure = options.Secure,
                     IsEssential = options.IsEssential,
-                    Expires = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc),
+                    Expires = DateTimeOffset.UnixEpoch,
                     HttpOnly = options.HttpOnly,
                 });
 
@@ -305,7 +305,7 @@ public void DeleteCookie(HttpContext context, string key, CookieOptions options)
                         SameSite = options.SameSite,
                         Secure = options.Secure,
                         IsEssential = options.IsEssential,
-                        Expires = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc),
+                        Expires = DateTimeOffset.UnixEpoch,
                         HttpOnly = options.HttpOnly,
                     });
             }