Remove alpine:3.11 docker image from ubuntu 18.04 and 20.04 (#4093)

Author: nikolai-frolov

images/linux/toolsets/toolset-1804.json

@@ -201,7 +201,6 @@
     ],
     "docker": {
         "images": [
-            "alpine:3.11",
             "alpine:3.12",
             "alpine:3.13",
             "alpine:3.14",

images/linux/toolsets/toolset-2004.json

@@ -200,7 +200,6 @@
     ],
     "docker": {
         "images": [
-            "alpine:3.11",
             "alpine:3.12",
             "alpine:3.13",
             "alpine:3.14",