Set datatable.old.matrix.autoname to FALSE by default

[MichaelChirico]

Continuation of #7158

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (be33aaa) to head (8ceefda).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7531   +/-   ##
=======================================
  Coverage   98.97%   98.97%           
=======================================
  Files          87       87           
  Lines       16732    16732           
=======================================
  Hits        16560    16560           
  Misses        172      172           

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[github-actions]

No obvious timing issues in HEAD=matrix-autoname-deprec

Generated via commit 8ceefda

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 45 seconds
Installing different package versions	43 seconds
Running and plotting the test cases	3 minutes and 36 seconds

[ben-schwen]

Basically LGTM