DM-32759: Remove support for the old "root" in config files

[timj]

No description provided.

[codecov]

Codecov Report

Merging #77 (9370464) into main (cc71245) will decrease coverage by 0.24%.
The diff coverage is 100.00%.

❗ Current head 9370464 differs from pull request most recent head 9406455. Consider uploading reports for the commit 9406455 to get more accurate results

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
- Coverage   84.10%   83.85%   -0.25%     
==========================================
  Files          40       40              
  Lines        3013     3004       -9     
==========================================
- Hits         2534     2519      -15     
- Misses        479      485       +6     

Impacted Files	Coverage Δ
python/lsst/pex/config/config.py	90.12% <100.00%> (-1.47%)	⬇️
tests/test_Config.py	95.35% <100.00%> (-0.12%)	⬇️

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