This updates the mpfr version to 3.1.6 which is the last bugfix
release from the 3.1.x series and avoids printing the version
is buggy but acceptable from our configury.

2021-05-17  Richard Biener  <rguenther@suse.de>

contrib/ChangeLog:
	* download_prerequisites: Update mpfr version to 3.1.6.
	* prerequisites.md5: Update.
	* prerequisites.sha512: Likewise.