Polymake.jl 0.5.7 released

Today Polymake.jl 0.5.7 has been released.

Diff since v0.5.6

Closed issues:

• Segmentation fault on Julia-1.7 beta3 (#351)

Merged pull requests:

• CI: adjust macos11 env name, remove old ubuntu (#350) (@benlorenz)
• Replace Array by Vector or Matrix where appropriate (#352) (@fingolfin)
• CI: add julia 1.7 and bump a few julia versions (#353) (@benlorenz)
• prepare for new release using flint 2.8 (#354) (@benlorenz)