Replaced deprecFix: Replace deprecated util.isArray with Array.isArray for Node.js compatibilityated util.isArray with Array.isArray

[VivekGhule]

Summary:
This pull request addresses a deprecation warning in Node.js: [DEP0044] DeprecationWarning: util.isArray is deprecated. The legacy util.isArray() function has been replaced with the modern Array.isArray() method to ensure compatibility with current and future Node.js versions.

Changes Made:

1. Removed var isArray = require('util').isArray.
2. Replaced all usages of isArray(msg) with Array.isArray(msg) in lib/flash.js.
3. No functional changes were made to existing flash message logic — behavior remains identical.

Benefits:

1. Eliminates the deprecation warning when running applications using connect-flash.
2. Ensures compatibility with Node.js v16+ and future releases.
3. Improves code maintainability by using standard, modern APIs.

Testing:

1. Verified that flash messages still work as expected in a local Express.js project.
2. Confirmed that the [DEP0044] warning no longer appears.

Additional Notes:
This is a backward-compatible change and does not affect any existing functionality of the package.