README: bump the gcc baseline to 8.4

README: bump the gcc baseline to 8.4

We already use __VA_OPT__ in multiple places, which was introduced in
gcc 8 [0], so let's bump the baseline to reflect that. I chose gcc 8.4,
as that was the lowest 8.x version I could easily get my hands on when I
verified this (on Ubuntu Focal with the gcc-8 package).

Closes: #31191
[0] https://gcc.gnu.org/gcc-8/changes.html