[JS] Add support for express 5.x in genkit express plugin

[jeffdh5]

Currently genkit express plugin depends on express 4.x. The latest version is express 5.x, which is pretty new, but something genkit should probably support eventually.

The best solution probably is to set the peerDependency to something like:

// Support both versions
{
  "peerDependencies": {
    "express": "^4.21.1 || ^5.0.0"
  }
}

We need to test it but at a glance I see that all the express features we're using are well-supported by express 5.x.