[macos] remove gcc9 from MacOS-11 (#7257)

Co-authored-by: Ilia Shipitsin <ilia-shipitsin@github.com>

Author: ilia-shipitsin

images/macos/toolsets/toolset-11.json

@@ -234,7 +234,6 @@
     },
     "gcc": {
         "versions": [
-            "9",
             "10",
             "11",
             "12"