CompatHelper: bump compat for IntervalArithmetic to 0.21, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the IntervalArithmetic package from 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20 to 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[vtjeng]

This is failing on Julia 1 (but not 1.6) since IntervalArithmetic:0.21.0 supports only Julia 1.8. See history of version bumps:

• 1.6 -> 1.7 in JuliaIntervals/IntervalArithmetic.jl@27a535e
• 1.7 -> 1.8 in JuliaIntervals/IntervalArithmetic.jl@91dede5

The failure seems to have to do with the error message:

   Got exception outside of a @test
  MethodError: no method matching IntervalArithmetic.Interval(::Float64, ::Float64)
  
  Closest candidates are:
    (::Type{T})(::T) where T<:Number
     @ Core boot.jl:792

• CI / Julia 1.6 - ubuntu-latest - x64 passes with IntervalArithmetic:0.20.9: https://github.com/vtjeng/MIPVerify.jl/actions/runs/5970175201/job/16197334077?pr=137#step:5:28
• CI / Julia 1 - ubuntu-latest - x64 fails with IntervalArithmetic:0.21.0: https://github.com/vtjeng/MIPVerify.jl/actions/runs/5970175201/job/16197334363?pr=137#step:6:46

There have been many changes between the two versions, so it may be challenging to bisect to find the issue. JuliaIntervals/IntervalArithmetic.jl@v0.20.9...v0.21.0

[vtjeng]

Superseded by #138.