cleanup support for Float32 and ComplexF32

[araujoms]

I fixed the same bug as #585, without realizing that it had already been done. It wasn't a complete waste of time, because the PR #586 left some cleanup to do: the lines

LinearAlgebra.qr(A::SparseMatrixCSC{<:Union{Float16,Float32}}; tol=_default_tol(A)) =
    qr(convert(SparseMatrixCSC{Float64}, A); tol=tol)
LinearAlgebra.qr(A::SparseMatrixCSC{<:Union{ComplexF16,ComplexF32}}; tol=_default_tol(A)) =
    qr(convert(SparseMatrixCSC{ComplexF64}, A); tol=tol)

would have prevented it from working if they actually worked! Luckily they don't, as the correct method is more specific and gets called, but it's still worth fixing. I also changed them to convert Float16 to Float32 instead of Float64, and the analogous change for complex numbers.

I also fixed the error messages one gets. They were of the form "dtype=$(s.dtype) not supported", which in my case translated to saying that Float64 wasn't supported when I tried calling it with Float32. s.dtype is the type that actually got computed, so it clearly is supported. I changed them to say "dtype=$(dtyp(Tv)) not supported".

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 84.08%. Comparing base (ff083ce) to head (ead6bd5).

Files with missing lines	Patch %	Lines
src/solvers/cholmod.jl	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #597      +/-   ##
==========================================
+ Coverage   84.06%   84.08%   +0.02%     
==========================================
  Files          12       12              
  Lines        9192     9192              
==========================================
+ Hits         7727     7729       +2     
+ Misses       1465     1463       -2     

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[ViralBShah]

LGTM. Let's give it a couple of days for comments.