contrib/download_prerequisites: Update GMP, MPFR, MPC [PR120237]

contrib/download_prerequisites: Update GMP, MPFR, MPC [PR120237]

Download newer versions of GMP, MPFR and MPC (the latest); besides the usual
bug fixes and smaller features, MPFR adds new functions for C23, some of
which are already used in GCC in the middle (fold-const-call.cc) and in
Fortran 2023 for the 'pi' trignonometric functions, if MPFR is new enough.

contrib/ChangeLog:

	PR other/120237
	* download_prerequisites: Update to download GMP 6.3.0 (before 6.2.1),
	MPFR 4.2.2 (before 4.1.0), and MPC 1.3.1 (before 1.2.1).
	* prerequisites.md5: Update hash.
	* prerequisites.sha512: Likewise.