legacy: move upgrade.py into borg.legacy, refs #9556

[mr-raj12]

Description

Moves UpgraderNoOp and UpgraderFrom12To20 into borg.legacy, refs #9556.

• src/borg/upgrade.py → src/borg/legacy/upgrade.py
• Relative imports updated from .X to ..X
• transfer_cmd.py: from .. import upgrade as upgrade_mod → from ..legacy import upgrade as upgrade_mod

No logic changes. Both classes are Borg 1.x-only. Leaving them at the package root means an extra file to track down when v1 support is dropped; in borg.legacy it goes away with the rest.

Refs #9556

Checklist

• PR is against master
• New code has tests
• Tests pass
• Commit message references related issue

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.08%. Comparing base (da0a2ff) to head (a3dacdc).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9655   +/-   ##
=======================================
  Coverage   83.08%   83.08%           
=======================================
  Files          90       90           
  Lines       15653    15653           
  Branches     2356     2356           
=======================================
  Hits        13006    13006           
  Misses       1887     1887           
  Partials      760      760           

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