Fix UTF-8 encoding errors introduced by #1801

!1801 (merged) introduced some hyphen characters in comments that aren't valid UTF-8, which caused MSVC and Apple Clang to complain.

D:\a\Sleipnir\Sleipnir\.py-build-cmake_cache\cp39-cp39-win_amd64\_deps\eigen3-src\Eigen\src\SparseCholesky\SimplicialCholesky_impl.h(1,1): error C2220: the following warning is treated as an error [D:\a\Sleipnir\Sleipnir\.py-build-cmake_cache\cp39-cp39-win_amd64\_jormungandr.vcxproj]
  (compiling source file '../../src/optimization/solver/InteriorPoint.cpp')
  
D:\a\Sleipnir\Sleipnir\.py-build-cmake_cache\cp39-cp39-win_amd64\_deps\eigen3-src\Eigen\src\SparseCholesky\SimplicialCholesky_impl.h(1,1): warning C4828: The file contains a character starting at offset 0x11fb that is illegal in the current source character set (codepage 65001). [D:\a\Sleipnir\Sleipnir\.py-build-cmake_cache\cp39-cp39-win_amd64\_jormungandr.vcxproj]
  (compiling source file '../../src/optimization/solver/InteriorPoint.cpp')

 In file included from /Users/runner/work/Sleipnir/Sleipnir/.py-build-cmake_cache/cp313-cp313-macosx_10_13_universal2/_deps/eigen3-src/Eigen/SparseCholesky:35:
/Users/runner/work/Sleipnir/Sleipnir/.py-build-cmake_cache/cp313-cp313-macosx_10_13_universal2/_deps/eigen3-src/Eigen/src/SparseCholesky/SimplicialCholesky_impl.h:130:52: error: invalid UTF-8 in comment [-Werror,-Winvalid-utf8]
  // ACM Trans. Math. Softw. 12, 2 (June 1986), 127<96>148. https://doi.org/10.1145/6497.6499
                                                   ^
/Users/runner/work/Sleipnir/Sleipnir/.py-build-cmake_cache/cp313-cp313-macosx_10_13_universal2/_deps/eigen3-src/Eigen/src/SparseCholesky/SimplicialCholesky_impl.h:200:67: error: invalid UTF-8 in comment [-Werror,-Winvalid-utf8]
  // SIAM Journal on Matrix Analysis and Applications, 15(4), 1075<96>1091.
                                                                  ^