relaxed eltype for ldiv! and rdiv! methods

[ArunS-tack]

updated diagonal.jl to relax eltype for ldiv! and rdiv! methods as suggested by @dkarrasch (#40942 (comment)).

ref: JuliaLang/LinearAlgebra.jl#849 , #40942

[dkarrasch]

These methods are probably rarely used (adjoint(D::Diagonal) returns a Diagonal, not an Adjoint{Diagonal}), but we should test this generalization anyway, perhaps along the lines of

A = rand(3,3)
for trans in (identity, Adjoint, Transpose)
    @test ldiv!(trans(I(3)), A) == A
    @test rdiv!(A, trans(I(3))) == A
end