Add changelog entry for deprecating mingw.

[MFraters]

Follow up to #744, adding a changelog entry.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	0.990 ± 0.004 (s=445)	0.990 ± 0.009 (s=467)	-0.2% .. +0.1%
Slab interpolation curved simple none	0.992 ± 0.008 (s=461)	0.992 ± 0.008 (s=448)	-0.2% .. +0.2%
Spherical slab interpolation simple none	1.165 ± 0.008 (s=404)	1.165 ± 0.008 (s=371)	-0.2% .. +0.1%
Slab interpolation simple curved CMS	1.032 ± 0.003 (s=436)	1.033 ± 0.004 (s=438)	-0.0% .. +0.1%
Spherical slab interpolation simple CMS	1.572 ± 0.013 (s=284)	1.569 ± 0.012 (s=292)	-0.4% .. +0.1%
Spherical fault interpolation simple none	1.165 ± 0.006 (s=382)	1.165 ± 0.007 (s=393)	-0.2% .. +0.1%
Cartesian min max surface	2.378 ± 0.026 (s=198)	2.379 ± 0.029 (s=183)	-0.4% .. +0.4%
Spherical min max surface	7.028 ± 0.035 (s=65)	7.030 ± 0.035 (s=66)	-0.3% .. +0.3%