Consider using "upgrade" instead of "migrate"

Daniel Roth suggested this change for ASP.NET Core docs in https://github.com/dotnet/AspNetCore.Docs/issues/28562.

From original (closed) PR:
I feel like this should be part of a larger PR that updates all the affected files, e.g. https://github.com/dotnet/docs/blob/main/docs/navigate/migration-guide/toc.yml, https://github.com/dotnet/docs/blob/main/docs/index.yml, and any affected articles under /docs/core/porting. Also perhaps the docs/navigate/migration-guide directory should be renamed (it only contains two files so redirecting isn't a big deal).

_Originally posted by @gewarren in https://github.com/dotnet/docs/pull/34399#pullrequestreview-1327069164_