Update app_events.exit_description to text

For databases that aren't Postgres, the "String" type in Sequel usually
maps to a varchar(255). This is problematic for the code pushed in
0c199d4 as it doesn't prevent SQL errors from being raised when the
payload exceeds 255 characters.

cloudfoundry/capi-release#152
cloudfoundry/capi-release#162

[#168010974]
[#170222110]

Authored-by: Aakash Shah <ashah@pivotal.io>

Author: Aakash Shah

db/migrations/20200507224709_change_crash_events_exit_description_to_text.rb

@@ -0,0 +1,7 @@
+Sequel.migration do
+  change do
+    alter_table(:app_events) do
+      set_column_type(:exit_description, 'text')
+    end
+  end
+end