[docs] switch from GLPK to HiGHS

[odow]

Closes #2878

TODOs

• check the preview. https://jump.dev/JuMP.jl/previews/PR2881

[codecov]

Codecov Report

Merging #2881 (36c680d) into master (1f20e03) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2881   +/-   ##
=======================================
  Coverage   95.25%   95.25%           
=======================================
  Files          43       43           
  Lines        5645     5645           
=======================================
  Hits         5377     5377           
  Misses        268      268           

Impacted Files	Coverage Δ
src/JuMP.jl	95.54% <ø> (ø)
src/lp_sensitivity2.jl	97.57% <ø> (ø)
src/optimizer_interface.jl	81.25% <ø> (ø)

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[odow]

@jajhall HiGHS is now the main solver we use in the JuMP documentation for MILPs.