[LinearAlgebra] Remove unreliable doctests (#56011)

The exact textual representation of the output of these doctests depend
on the specific kernel used by the BLAS backend, and can vary between
versions of OpenBLAS (as it did in #41973), or between different CPUs,
which makes these doctests unreliable.

Fix #55998.

stdlib/LinearAlgebra/src/LinearAlgebra.jl

@@ -394,17 +394,8 @@ julia> Y = zero(X);
 
 julia> ldiv!(Y, qr(A), X);
 
-julia> Y
-3-element Vector{Float64}:
-  0.7128099173553719
- -0.051652892561983674
-  0.10020661157024757
-
-julia> A\\X
-3-element Vector{Float64}:
-  0.7128099173553719
- -0.05165289256198333
-  0.10020661157024785
+julia> Y ≈ A\\X
+true
 ```
 """
 ldiv!(Y, A, B)
@@ -435,17 +426,8 @@ julia> Y = copy(X);
 
 julia> ldiv!(qr(A), X);
 
-julia> X
-3-element Vector{Float64}:
-  0.7128099173553719
- -0.051652892561983674
-  0.10020661157024757
-
-julia> A\\Y
-3-element Vector{Float64}:
-  0.7128099173553719
- -0.05165289256198333
-  0.10020661157024785
+julia> X ≈ A\\Y
+true
 ```
 """
 ldiv!(A, B)

stdlib/LinearAlgebra/src/hessenberg.jl

@@ -446,7 +446,7 @@ This is useful because multiple shifted solves `(F + μ*I) \\ b`
 Iterating the decomposition produces the factors `F.Q, F.H, F.μ`.
 
 # Examples
-```jldoctest
+```julia-repl
 julia> A = [4. 9. 7.; 4. 4. 1.; 4. 3. 2.]
 3×3 Matrix{Float64}:
  4.0  9.0  7.0