Deprecate implicit pseudo-inverses of vectors

[antoine-levitt]

See #49915. If we can't remove it, at least let's deprecate it. I'm still not super happy about it because deprecations aren't shown by default, but it's better than nothing.

Note that this now deprecates all pseudo-inverses, eg vector \ vector, unlike the previous PR that just removed real / vector.

[brenhinkeller]

I don't know if there's really much point adding deprecations for breaking changes that we can't make
Eh, I guess I take that back if there's no performance implication and there's general agreement this is wanted in 2.0 and adding this now makes that easier

[antoine-levitt]

We can still send a signal "this function was a mistake, don't use it, and hypothetically if we ever get around to making breaking changes we'll remove it"

[antoine-levitt]

Can we merge this?

[DilumAluthge]

We have moved the LinearAlgebra stdlib to an external repo: https://github.com/JuliaLang/LinearAlgebra.jl

@antoine-levitt If you think that this PR is still relevant, please open a new PR on the LinearAlgebra.jl repo.