Fix precompilation warnings

[j-fu]

See e.g. https://discourse.julialang.org/t/warning-about-importing-spksparsespdbase-triangularsolve/128195

EDIT: Fixes the following warnings on Julia 1.12 beta:

2 dependencies successfully precompiled in 3 seconds. 12 already precompiled.
1 dependency had output during precompilation:
┌ Sparspak
│  WARNING: Imported binding SpkSparseSpdBase._triangularsolve! was undeclared at import time during import to SpkSparseSpdSolver.
│  WARNING: Constructor for type "Graph" was extended in `SparseCSCInterface` without explicit qualification or import.
│    NOTE: Assumed "Graph" refers to `SpkGraph.Graph`. This behavior is deprecated and may differ in future versions.`
│    NOTE: This behavior may have differed in Julia versions prior to 1.12.
│    Hint: If you intended to create a new generic function of the same name, use `function Graph end`.
│    Hint: To silence the warning, qualify `Graph` as `SpkGraph.Graph` in the method signature or explicitly `import SpkGraph: Graph`.
│  WARNING: Constructor for type "_SparseBase" was extended in `SparseCSCInterface` without explicit qualification or import.
│    NOTE: Assumed "_SparseBase" refers to `SpkSparseBase._SparseBase`. This behavior is deprecated and may differ in future versions.`
│    NOTE: This behavior may have differed in Julia versions prior to 1.12.
│    Hint: If you intended to create a new generic function of the same name, use `function _SparseBase end`.
│    Hint: To silence the warning, qualify `_SparseBase` as `SpkSparseBase._SparseBase` in the method signature or explicitly `import SpkSparseBase: _SparseBase`.
│  WARNING: Constructor for type "SparseSolver" was extended in `SparseCSCInterface` without explicit qualification or import.
│    NOTE: Assumed "SparseSolver" refers to `SpkSparseSolver.SparseSolver`. Thisbehavior is deprecated and may differ in future versions.`
│    NOTE: This behavior may have differed in Julia versions prior to 1.12.
│    Hint: If you intended to create a new generic function of the same name, use `function SparseSolver end`.
│    Hint: To silence the warning, qualify `SparseSolver` as `SpkSparseSolver.SparseSolver` in the method signature or explicitly `import SpkSparseSolver: SparseSolver`.
└

[j-fu]

@PetrKryslUCSD: MacOS14 cannot be tested on 1.6. May be we bump the min version to be tested on to 1.9 ?