Deprecate some DGMultiMesh constructors

[jlchan]

One deprecated DGMultiMesh constructor remains:

# TODO: deprecations introduced in Trixi.jl v0.6
@deprecate DGMultiMesh(dg::DGMulti{NDIMS}; cells_per_dimension, kwargs...) where {NDIMS} DGMultiMesh(dg,
                                                                                                     cells_per_dimension;
                                                                                                     kwargs...)

However, since this was deprecated in the v0.5 lifecycle, I didn't remove it. Should it be removed, or should we leave this until v0.7?

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c79fa43) 72.65% compared to head (6b03020) 75.13%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1709      +/-   ##
==========================================
+ Coverage   72.65%   75.13%   +2.48%     
==========================================
  Files         431      431              
  Lines       34637    34657      +20     
==========================================
+ Hits        25164    26039     +875     
+ Misses       9473     8618     -855     

Flag	Coverage Δ
unittests	75.13% <ø> (+2.48%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/solvers/dgmulti/types.jl	98.12% <ø> (ø)

... and 60 files with indirect coverage changes

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[sloede]

Should it be removed, or should we leave this until v0.7?

I think it's OK to remove it. As the Germans say, we don't need to be more pontifical than the pope. Objections @ranocha @andrewwinters5000?

[andrewwinters5000]

I think it is okay to remove it as well.

[ranocha]

Yes, we can remove it in this breaking change 👍

[ranocha]

Thanks!

[jlchan]

Merging as per comments on Slack