Statistics: update Flotr2 to latest release from EnterpriseDB (#3800)

It contains a few minor fixes for Google's breaking changes in Blink.

See <https://github.com/EnterpriseDB/Flotr2> for the repo.

Cf. <https://github.com/FreshRSS/FreshRSS/pull/3799#issuecomment-905370969>