Update Gutenboarding to use new <LocaleProvider>

[p-jackson]

Split from #47446

Changes proposed in this Pull Request

• Switch gutenboarding from getting the locale slug from react-i18n to i18n-utils

For the reasons outlined in #47566 the locale slug shouldn't come from react-i18n. This PR migrates gutenboarding away from that package.

Testing instructions

Test that all the gutenboarding locale features are working just like before

• The UI at /new should be in the current users language
• /new/fr should show the UI in french
• The plans grid and domain picker should be translated correctly
• Using an incognito window, go throught the flow starting from /new/fr and create a new user. The language of the new account should be set to French

[matticbot]

Test live: https://calypso.live/?branch=update/gutenboarding-use-locale-provider-instead-of-react-i18n

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~190 bytes added 📈 [gzipped])

name                 parsed_size           gzip_size
entry-gutenboarding       +652 B  (+0.0%)     +190 B  (+0.0%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[lsl]

LGTM depends deploy of #47570 so we avoid pulling i18n-calypso to anything we don't want to.

[lsl]

#47570 deployed, probably a good idea to rebase this