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

@@ -2,46 +2,50 @@
 # PRODUCTION
 #
 production:
-  adapter: postgresql
-  host: /var/run/postgresql
-  encoding: unicode
-  database: gitlab_production
-  pool: 10
-  # username: git
-  # password:
-  # host: localhost
-  # port: 5432 
+  main:
+    adapter: postgresql
+    host: /var/run/postgresql
+    encoding: unicode
+    database: gitlab_production
+    pool: 10
+    # username: git
+    # password:
+    # host: localhost
+    # port: 5432
 
 #
 # Development specific
 #
 development:
-  adapter: postgresql
-  encoding: unicode
-  database: gitlabhq_development
-  pool: 5
-  username: postgres
-  password:
+  main:
+    adapter: postgresql
+    encoding: unicode
+    database: gitlabhq_development
+    pool: 5
+    username: postgres
+    password:
 
 #
 # Staging specific
 #
 staging:
-  adapter: postgresql
-  encoding: unicode
-  database: gitlabhq_staging
-  pool: 5
-  username: postgres
-  password:
+  main:
+    adapter: postgresql
+    encoding: unicode
+    database: gitlabhq_staging
+    pool: 5
+    username: postgres
+    password:
 
 # Warning: The database defined as "test" will be erased and
 # re-generated from your development database when you run "rake".
 # Do not set this db to the same as development or production.
 test: &test
-  adapter: postgresql
-  encoding: unicode
-  host: /var/run/postgresql
-  database: gitlab_test
-  pool: 5
-  #username: postgres
-  password:
+  main:
+    adapter: postgresql
+    encoding: unicode
+    host: /var/run/postgresql
+    database: gitlab_test
+    pool: 5
+    #username: postgres
+    password: