Turn on countdownForCooldowns when migrating the old cooldown settings

pfUI drew its own cooldown numbers before; the native countdown shows
them only with the countdownForCooldowns cvar on, and only the first-run
setup set it. Upgraded installs showed no numbers at all, so the
cooldown settings migration turns the cvar on as it removes
appearance.cd.

api/config.lua

@@ -1453,6 +1453,10 @@ function pfUI:MigrateConfig()
       cooldown.blizzard.text = cooldown.foreign.text
     end
 
+    -- pfUI drew its own numbers before; the native countdown shows them
+    -- only with this cvar on, which only the first-run setup sets
+    SetCVar("countdownForCooldowns", "1")
+
     pfUI_config.appearance.cd = nil
   end