Update/connection banner multisite url redirection

[darssen]

Fixes VULCAN-101

Proposed changes:

• Replace not only wp-admin but all before the first occurrence when getting the redirect_uri in the ConnectBanner
  The replace code was introduced in Add/skip pricing page when connecting from editor blocks #39865 but didn't consider multisites.

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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

Same as #39865
And also #43340

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Code Coverage Summary

This PR did not change code coverage!

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[darssen]

Definitely not familiar with the codebase, but I think this started being an issue with #39865, which didn't consider multisites when replacing only wp-admin/ I'm not 100% sure if that would be an issue for other places where the component is used although I doubt it.

I opted for a regex in the same ConnectBanner to avoid other issues, but another option would be to be more defensive down the road here and check for multisites and replace the site_url. I think it's not the best option.

[coder-karen]

Nice!!
This works well, thank you.