Create a release-1.8 branch and backport fixes to it

[KristofferC]

Stdlibs that live in their own repo generally needs to do a similar backport process as in the Julia repo. Roughly this is done by:

• Have a release-1.x branch.
• Before a release, create a backports-release-1.x branch.
• Backport fixes on master that are suitable for backporting to backports-release-1.x
• When CI looks good on that branch, merge it into release-1.x.
• Bump the stdlib/SparseArrays.version on the backport branch in the Julia repo.

[rayegun]

To run CI on that branch I have to PR from it correct? PR from backports-release-1.8 to release-1.8. And then we can also collect the solution to #251 as well hopefully.

[DilumAluthge]

• Bump the stdlib/SparseArrays.version on the backport branch in the Julia repo.

Note: currently, this step needs to be done manually. However, once JuliaLang/BumpStdlibs.jl#76 has been done, we'll be able to use the BumpStdlibs bot to automate this step.