Switch Requires.jl to weakdep, and require Julia 1.10+ (thus dropping support for Julia 1.6 through 1.9)

[oscardssmith]

julia> @time using Quadmath
  0.009195 seconds (8.15 k allocations: 549.961 KiB, 34.84% compilation time) # before
  0.006442 seconds (4.01 k allocations: 305.828 KiB) #after

[DilumAluthge]

Looks like Windows CI is failing?

[DilumAluthge]

Is this change (dropping support for Julia 1.6 through 1.9) intentional?

[oscardssmith]

yes (but I can make this work on older versions if desired)

[DilumAluthge]

Actually, I don't even think we need to drop support for Julia 1.6 through 1.9. Instead, we can do the following:

1. On Julia versions 1.6 through 1.9, make SpecialFunctions a regular direct dep.
2. On Julia 1.10+, make SpecialFunctions a weakdep.

Basically, following the instructions in the "Transition from normal dependency to extension" section here: https://pkgdocs.julialang.org/v1/creating-packages/#Transition-from-normal-dependency-to-extension

[DilumAluthge]

And then, since we aren't dropping support for any old Julia versions, we can just do the patch version bump of 0.5.13 ---> 0.5.14.

[DilumAluthge]

Either way, we do need to fix Windows CI on this PR.