BLD: update minimum required GCC to GCC 10 #31828

See #28102, which was the last time we bumped the minimum required GCC version 18 months ago.

According to https://scipy.github.io/devdocs/dev/toolchain.html#compilers, GCC 9 is still the minimum required for both NumPy and SciPy.

Over in #31822 we are hitting CI failures that happen only with GCC 9.

GCC 9 ships with Ubuntu 20.04 (six years old and EOL now) and is not needed on any of the manylinux images we use, they need GCC 10 or newer.

@rgommers what do you think? This isn't motivated by anything besides seeing CI breakage which we can work around. Then again, it does seem like a reasonable time to do this bump for a version of GCC that came out in 2019.