fix(markdown): drop deprecated --highlight-style

[sxl613]

pandoc 3.8 deprecated --highlight-style in favour of --syntax-highlighting, so newer pandoc prints:

Deprecated: --highlight-style. Use --syntax-highlighting instead.

+markdown-compile-pandoc passes a plain buffer as call-process-region's BUFFER argument, which merges stderr into stdout, so this warning is injected into the rendered HTML instead of being discarded. Seen with pandoc 3.10.1 via markdown-preview.

Rather than gate the flag on pandoc <3.8 vs >=3.8, drop it altogether: pygments is already pandoc's default style, so output is unchanged and the command still works on every pandoc version.

Ref: https://github.com/jgm/pandoc/releases/tag/3.8
specifically

• Add a new command line option --syntax-highlighting; this takes
  the values none, default, idiomatic, a style name, or a path to a
  theme file. It replaces the --no-highlighting, --highlighting-style,
  and --listings options, which will still work but with a deprecation
  warning. (Albert Krewinkel)

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