Upgrade Gitea to 1.15.6

Configures the JWT signing algorithm to be the old, less secure
algorithm, until we update the token for Drone CI (and any other OAuth
apps).

closes #338

site-cookbooks/kosmos_gitea/templates/default/app.ini.erb

@@ -46,6 +46,7 @@ PASSWD = <%= @smtp_password %>
 
 [oauth2]
 JWT_SECRET = <%= @jwt_secret %>
+JWT_SIGNING_ALGORITHM = HS256
 
 [security]
 INTERNAL_TOKEN = <%= @internal_token %>