Replace GlobalConfiguration.DefaultCulture with CultureInfo.CurrentCulture

[tmilnthorp]

Replace GlobalConfiguration.DefaultCulture with CultureInfo.CurrentCulture. Breaking change so belongs in #563.

• CultureInfo.CurrentCulture for number formatting
• Use CultureInfo.CurrentCulture or CultureInfo.CurrentUICulture for abbreviations?

[angularsen]

Can we perhaps introduce this by using CultureInfo.CurrentCulture as the backing value and mark the class as obsolete, then remove it in v5?

[tmilnthorp]

Good idea 👍