Make Bootstrap4 the default UIFramework for the Identity UI

[Eilon]

Change this to Bootstrap4:

https://github.com/aspnet/AspNetCore/blob/bfec2c14be1e65f7dd361a43950d4c848ad0cd35/src/Identity/UI/src/IdentityBuilderUIExtensions.cs#L33

This will help avoid all the places in the templates where the value must be set to Bootstrap4 explicitly, such as:

https://github.com/aspnet/AspNetCore/blob/bc009fc0f180532e003b7d3e466c5a1a2456a630/src/ProjectTemplates/Web.ProjectTemplates/content/RazorPagesWeb-CSharp/Startup.cs#L73

Note: This is super Announcement-worthy, so please make sure an announcement issue gets logged for this.

So, the work here is to:

• Make the default Bootstrap4
• Remove the setting from all the templates
• Post an announcement

[Eilon]

Please remember to post an announcement as well.

[ryanbrandenburg]

Created #9509 to track that.