Clearly show a warning when attempting to use BLAS.vendor()

[ViralBShah]

With the move to using LBT, we should mark BLAS.vendor() as deprecated for 1.7. It is practically not used across the package ecosystem (discussed in JuliaLang/julia#39455). And since it is a private API, we should then be able to remove it altogether in 1.8.

Otherwise, having it creates confusion: JuliaLang/julia#33691 (comment), JuliaLinearAlgebra/MKL.jl#76

[giordano]

It is already deprecated:
https://github.com/JuliaLang/julia/blob/c3a31804a88beb4b05c8fd4b0981e76d14058311/stdlib/LinearAlgebra/src/blas.jl#L94
The problem is probably that deprecation warnings are hidden by default?

[ViralBShah]

That is indeed the problem. Maybe we need to print a warning in addition to the depwarn to make sure that people see the warning? It's largely used interactively by users rather than in packages, based on a pkgeval run.

[simeonschaub]

I think we could just pass force=true to Base.depwarn, which should cause the warning to always show up.

[ViralBShah]

That would certainly solve the problem.