Update unit_tests.yml to disable 5.9 by default (#56)

Update unit_tests.yml to disable 5.9 by default

Author: rnro

.github/workflows/unit_tests.yml

@@ -5,8 +5,8 @@ on:
     inputs:
       linux_5_9_enabled:
         type: boolean
-        description: "Boolean to enable the Linux 5.9 Swift version matrix job. Defaults to true."
-        default: true
+        description: "Boolean to enable the Linux 5.9 Swift version matrix job. Defaults to false."
+        default: false
       linux_5_9_arguments_override:
         type: string
         description: "The arguments passed to swift test in the Linux 5.9 Swift version matrix job."