Update syntax of database config

The format we used for the database config YAML file is deprecated, and
will no longer be supported starting with GitLab 15.0.
cf. https://gitlab.com/gitlab-org/gitlab/-/issues/338182

The new format is described in the upstream documentation:
https://docs.gitlab.com/ee/development/database/multiple_databases.html#configdatabaseyml

debian/conf/database.yml

@@ -2,6 +2,7 @@
 # PRODUCTION
 #
 production:
+  main:
     adapter: postgresql
     host: /var/run/postgresql
     encoding: unicode
@@ -16,6 +17,7 @@ production:
 # Development specific
 #
 development:
+  main:
     adapter: postgresql
     encoding: unicode
     database: gitlabhq_development
@@ -27,6 +29,7 @@ development:
 # Staging specific
 #
 staging:
+  main:
     adapter: postgresql
     encoding: unicode
     database: gitlabhq_staging
@@ -38,6 +41,7 @@ staging:
 # re-generated from your development database when you run "rake".
 # Do not set this db to the same as development or production.
 test: &test
+  main:
     adapter: postgresql
     encoding: unicode
     host: /var/run/postgresql