Default Bootstrap version of IdentityUI changed to 4

## Default Bootstrap version of IdentityUI changed to 4

Currently the default version of Bootstrap for IdentityUI is `UIFramework.Bootstrap3`, but starting in 3.0 the default will be `UIFramework.Bootstrap4`.

### Version introduced

3.0

### Old behavior

The `services.AddDefaultIdentity<IdentityUser>().AddDefaultUI();` would be the same as `services.AddDefaultIdentity<IdentityUser>().AddDefaultUI(UIFramework.Bootstrap3);`

### New behavior

The `services.AddDefaultIdentity<IdentityUser>().AddDefaultUI();` would be the same as `services.AddDefaultIdentity<IdentityUser>().AddDefaultUI(UIFramework.Bootstrap4);`

### Reason for change

Bootstrap 4 was released during 3.0 time frame

### Recommended action

If you happen to use the Defautl IdentityUI, and have wired it up in your `Startup` as shown below, then you'll be impacted by this change:

```csharp
services.AddDefaultIdentity<IdentityUser>().AddDefaultUI();
```

You can either migrate your application to use Bootstrap 4 using their [migration guide](https://getbootstrap.com/docs/4.0/migration), or simply update your code as follows to enforce usage of Bootstrap 3:

```csharp
services.AddDefaultIdentity<IdentityUser>().AddDefaultUI(UIFramework.Bootstrap3);
```

### Category

ASP.NET Core

### Affected APIs

     "Not detectable via API analysis"



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#### Issue metadata

* Issue type: breaking-change

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Default Bootstrap version of IdentityUI changed to 4 #14882