Bump minimum gcc to 8.4.

8.3 did not work with tests based on std::filesystem:

Bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90050

Even linking with -lstdc++fs did not help. 8.4 works, 9.2.1 too.

web/data/versions.json

 {
   "minimum_version": {
-    "gcc": "8.0",
+    "gcc": "8.4",
     "clang": "9.0",
     "msvc": {
       "year": "2019",