databases/mysq57-{client, server}: Update to latest release 5.7.31

Full Changelog: https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-31.html

Submitted by: fluffy
MFH: 2020Q3
Security: 0ed71663-c369-11ea-b53c-d4c9ef517024
Sponsored by: Netzkommune GmbH