move out LinearAlgebra into its own repository (#56637)
This moves out LinearAlgebra into its own repo
https://github.com/JuliaLang/LinearAlgebra.jl. This repo is still a bit
bare (README needs to be added) but it has CI set up to run on buildkite
(https://buildkite.com/julialang/linearalgebra-dot-jl/builds/18) and doc
building on GHA. The external repo has all commits up to
https://github.com/JuliaLang/julia/commit/4709b6c48e79f6226e6dbee1b49bf7e563058ff7
included in it.
The reason for the move is to be able to focus issues and PRs and
development regarding LinearAlgebra in one place.
