Update mpfr version to 3.1.6

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.