Update Builders.Functions.fs

[kumkee]

Related to https://github.com/CompositionalIT/farmer/issues/1031 - default ExtensionVersion: V3 -> V4

This PR closes #1031

The changes in this PR are as follows:

• In the Functions Builder default ExtensionVersion: V3 -> V4

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [x] Updated the documentation in the docs folder for the affected changes.
• [ ] Written unit tests against the modified code that I have made.
• [ ] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here: This is rather a tiny change.

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

open Farmer
open Farmer.Builders

let func =
    functions {
        name "myBareFunction"
        // use_extension_version V4 // -> V4 is officially recommended. With the PR, this line can be removed.
    }

let deployment =
    arm {
        location Location.AustraliaEast
        add_resource func
    }

deployment |> Deploy.execute "myBareFunction_grp" Deploy.NoParameters |> ignore

[ninjarobot]

@kumkee I had to fix the regression test to match with the code change.

[ninjarobot]

@isaacabraham any concerns with switching the default extension version to V4 now?