fix: remove country code for ubuntu

[dhoppe]

Summary of Pull Request

The uri should not contain a country code like us. Otherwise the GeoIP lookup does not work.

deb http://us.archive.ubuntu.com/ubuntu jammy main restricted
vs
deb http://de.archive.ubuntu.com/ubuntu jammy main restricted

https://github.com/canonical/subiquity/blob/main/documentation/autoinstall-reference.md?plain=1#L214

Type of Pull Request

• This is a bugfix. type/bug
• This is an enhancement or feature. type/feature or type/enhancement
• This is a documentation update. type/docs
• This is a refactoring update. type/refactor
• This is a chore. type/chore
• This is something else.
  Please describe:

Related to Existing Issues

Issue Number: N/A

Test and Documentation Coverage

• Tests have been completed.
• Documentation has been added or updated.

Breaking Changes?

• Yes, there are breaking changes.
• No, there are no breaking changes.

[tenthirtyam]

LGTM! 🚀

[github-actions]

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