repo: site_cache_dir: take major version into account

[efiop]

So that different major dvc versions don't use the same cache path.

Would be nice to also take creation time into account, but it is not universally exposed yet (only for macos it is as simple as os.stat(path).st_birthtime, so might add it there later).

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (d69972d) 90.53% compared to head (966d281) 90.53%.

❗ Current head 966d281 differs from pull request most recent head 93b2f01. Consider uploading reports for the commit 93b2f01 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9699   +/-   ##
=======================================
  Coverage   90.53%   90.53%           
=======================================
  Files         480      480           
  Lines       36420    36421    +1     
  Branches     5232     5232           
=======================================
+ Hits        32974    32975    +1     
  Misses       2855     2855           
  Partials      591      591           

Impacted Files	Coverage Δ
dvc/repo/__init__.py	95.04% <100.00%> (+0.01%)	⬆️

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