chore(ci): treat HTTP 429 as "found" for linkcheck (#4016)

Modifies the configuration of the dead link checker so that it does not
fail when HTTP 429 is returned (too many requests). This should
alleviate some of the spuriously failing builds we have been seeing.

Signed-off-by: Eric Chlebek <echlebek@sumologic.com>

Author: echlebek

.markdown_link_check.json

@@ -18,5 +18,6 @@
     {
       "pattern": "^https://stackoverflow.com/"
     }
-  ]
+  ],
+  "aliveStatusCodes": [200, 206, 300, 301, 302, 303, 304, 307, 308, 429]
 }