cache: remove try_upgrade_to_b14() legacy migration, fixes #9371

[mr-raj12]

Summary

• Remove try_upgrade_to_b14() from src/borg/cache.py — a one-time migration function marked # TODO: remove this before 2.0.0 release
• Added in b14 (d4310dd, Nov 2024) to invalidate old chunk index caches after the refcount → flags rename and CHUNKINDEX_HASH_SEED change
• Any repo used since b14 has already been cleaned up — the function is now a no-op that always hits the except branch
• It runs on every build_chunkindex_from_repo() call, issuing 2 unnecessary store_delete() RPCs per cache rebuild
• Old caches are also rejected at the hash-validation level in read_chunkindex_from_repo_cache(), so this function provides no safety net

Fixes #9371

Changes

File	Change
src/borg/cache.py	Removed try_upgrade_to_b14() (lines 697–705) and its call in build_chunkindex_from_repo() (line 810)

Checklist

• PR is against master
• Single-topic changeset — removal only, no mixed refactoring
• Code formatted with black

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.44%. Comparing base (c7261ae) to head (cace06f).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9372   +/-   ##
=======================================
  Coverage   76.44%   76.44%           
=======================================
  Files          85       85           
  Lines       14806    14799    -7     
  Branches     2213     2212    -1     
=======================================
- Hits        11318    11313    -5     
+ Misses       2810     2808    -2     
  Partials      678      678           

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[ThomasWaldmann]

Thanks!