Update default tag for Phabricator tickets

Since Gitlab issues are being used widely, not only for bugs, let's make
it clear and change the default tag in Phabricator. Later, managers
can add additional tags to better describe the ticket.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

config.yaml

@@ -3,7 +3,7 @@ areas:
   - infrastructure
   - test-failure
 phabricator-projects:
-  - bug
+  - public-issue
 all-issues-managed: True
 user-map:
   not_a_robot: apertis-ci