Test CI with master, 1, and LTS

[ViralBShah]

We are currently testing only master, but presumably we should test both the latest stable release as well as LTS. In SuiteSparse, we enabled latest stable testing.

@DilumAluthge Are there any best practices here?

[KristofferC]

In Pkg, we test master Pkg only on master. And then we have release-1.6, release-1.7 branches that we backport to that run tests on the respective Julia version.

[DilumAluthge]

On the subject of backporting, would it be useful for us to install a backport GitHub Action on this repository?

For the main JuliaLang/julia rapidity, I think the current system (where Kristoffer manually runs the https://github.com/KristofferC/Backporter script on his local machine) works very well. However, for stdlibs that live in external repos, I don't know if this is scalable, since now Kristoffer will have to run the script N + 1 times (if there are N stdlibs that live in external repos).

So, for some of these external stdlibs, perhaps we could install a backport GitHub Action, and use that instead? For example: https://github.com/tibdex/backport

[DilumAluthge]

Here's a PR to install a backport GitHub Action: #62

[ViralBShah]

Will these backports go to julialang/Julia?

[KristofferC]

No, they would go to a release-1.6, release-1.7 branch on this repo. But to create them we would need to do the same thing as we did for the master branch here (extract all commits that affected SparseArrays) but do it for the Juliarelease-1.6, release-1.7 branches. I don't think that should be too difficult.