plots: disable syntax highlighting on plots json output

[skshetry]

I don't think it's worth it to highlight given the size of json output.

Note that highlighting was/is off when the stdout is redirected.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.32 ⚠️

Comparison is base (aaa825a) 92.92% compared to head (48d8967) 92.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9155      +/-   ##
==========================================
- Coverage   92.92%   92.61%   -0.32%     
==========================================
  Files         456      456              
  Lines       36892    36899       +7     
  Branches     5322     5325       +3     
==========================================
- Hits        34281    34173     -108     
- Misses       2088     2181      +93     
- Partials      523      545      +22     

Impacted Files	Coverage Δ
dvc/commands/plots.py	88.95% <100.00%> (ø)
dvc/ui/__init__.py	83.33% <100.00%> (+1.82%)	⬆️

... and 30 files with indirect coverage changes

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