webapp: use the postgresql replica to avoid timeouts

There were no timeouts since the manual update this morning

Test Plan

diff origin/production/moma.softwareheritage.org current/moma.softwareheritage.org
*******************************************
  File[/etc/softwareheritage/storage/storage.yml] =>
   parameters =>
     content =>
      @@ -3,5 +3,5 @@
         cls: local
         args:
      -    db: host=belvedere.internal.softwareheritage.org port=5432 user=guest dbname=softwareheritage
      +    db: host=somerset.internal.softwareheritage.org port=5432 user=guest dbname=softwareheritage
             password=guest
           objstorage:
*******************************************
  File_line[swhwebapp:pgpass:softwareheritage-sync-mailmaps] =>
   parameters =>
     line =>
      - belvedere.internal.softwareheritage.org:5432:softwareheritage:swhmailmap:swh-deploy-webapp-sync_mailmaps-db-password
      + somerset.internal.softwareheritage.org:5432:softwareheritage:swhmailmap:swh-deploy-webapp-sync_mailmaps-db-password
     match =>
      - ^belvedere.internal.softwareheritage.org:5432:softwareheritage:swhmailmap:
      + ^somerset.internal.softwareheritage.org:5432:softwareheritage:swhmailmap:
*******************************************
  Ini_setting[swhwebapp:pg_service.conf:softwareheritage-sync-mailmaps:host] =>
   parameters =>
     value =>
      - belvedere.internal.softwareheritage.org
      + somerset.internal.softwareheritage.org
*******************************************
*** End octocatalog-diff on moma.softwareheritage.org

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Migrated from D7340 (view on Phabricator)