Use g++9 on Ubuntu 19.10/20.04 LTS+ instead of g++8
Overview
We can now safely g++9
package on the most Ubuntu environments (eoan (19.10), focal (20.04), or higher) while we came to enforce using g++8
over g++7
on xenial (16.04) or bionic (18.04) on !1087 (merged).
Proposal (optional)
Specifying g++8
only on bionic or lower on docs is required. Actually GCC 8.x was introduced in 18.10.
Edited by Takuya Noguchi