T7019: use VyOS release train in /etc/os-release codename over Debian release

[c-po]

VyOS is based on Debian

• VyOS 1.3 -> Debian Buster (VyOS equuleus)
• VyOS 1.4 -> Debian Bookworm (VyOS sagitta)
• VyOS 1.5 -> Debian Bookworm (and then trixie) (VyOS circinus)
• VyOS rolling -> Debian Bookworm (and then trixie) (VyOS t.b.d.)

When running

  vyos@vyos:~$ lsb_release -a Distributor ID: VyOS
  Description:    VyOS 1.5-rolling-202501031241 (current)
  Release:        1.5-rolling-202501031241
  Codename:       bookworm

The codename in use is the Debian base distribution. This should be changed to the VyOS release name.

Change summary

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):

Related Task(s)

• https://vyos.dev/T7019

Related PR(s)

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

[github-actions]

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