Update migration instructions for payment providers

[CarlSargunar]

Updated instructions on the upgrade from Vendr to Umbraco Commerce to fix issues with the SQL

• sp_rename was erroring when I ran it, corrected the syntax
• Corrected namespace on custom providers
• Fixed an issue with the data type of a colum - was nText, which was deprecated, had to convert to nvarchar(max)

[CarlSargunar]

Additional changes to add : Vendr.Services.CurrencyService.GetCurrency(ctx.Order.CurrencyId); to be replaced with injected ICurrencyService

Vendr.Services.CountryService.GetCountry(ctx.Order.PaymentInfo.CountryId.Value) to be replaced with injected ICountryService

[CarlSargunar]

And IVendrBuilder to IUmbracoCommerceBuilder

[CarlSargunar]

Also mention about ModelsBuilder - eg Homepage node has a reference to Store, but the namespaces no longer exist.

	///<summary>
	/// Store
	///</summary>
	[global::System.CodeDom.Compiler.GeneratedCodeAttribute("Umbraco.ModelsBuilder.Embedded", "10.8.11+d8f68d2")]
	[global::System.Diagnostics.CodeAnalysis.MaybeNull]
	[ImplementPropertyType("store")]
	public virtual global::Umbraco.Commerce.Core.Models.StoreReadOnly Store => this.Value<global::Umbraco.Commerce.Core.Models.StoreReadOnly>(_publishedValueFallback, "store");

[CarlSargunar]

Changes to Order.AddProduct, writeableOrder.AddProduct

[mattbrailsford]

The changes mostly look good (have given feedback on what needs changing), but the PR discussion mentions other changes needed that aren't part of this PR. These either need implementing, or moving to an Issue so that they don't get lost when the PR is merged.

[mattbrailsford]

Should probably just be Vendr

[mattbrailsford]

Should probably just be Vendr

[CarlSargunar]

The note about other places was to myself really to remember to change it for the namespaces. There were some bits about IVendrBuilder to IUmbracoCommerceBuilder which I can add if you're happy to hold on for a bit

Also thanks for your help resolving that product adapter issue :-)