T6755: Change default vyos mirror URL (backport #780)

[mergify]

Change Summary

Change default vyos mirror URL

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe): change default vyos mirror URL

Related Task(s)

• https://vyos.dev/T6755

Related PRs

vyos/vyos-1x#4119

Component(s) name

Proposed changes

How to test

build iso image

[2024-10-02 07:27:08] lb source 
P: Source stage disabled, skipping
P: Build completed successfully
vyos_bld@2595ca2a1064:/vyos/work/tmp/vyos-build$

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #780 done by [Mergify](https://mergify.com).

[github-actions]

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