Update "software architecture" image

The image here needs some updates/fixes:

1. swh-web now uses a pg database instead of sqlite
2. there should be a journal client for the scheduler, in addition to the idx journal client
3. idx workers should have an arrow to the idx storage, objstorage. And either the idx workers or idx-storage should have an arrow to the journal

But the last two points would make the diagram even more complex than it already is.

So I think we should remove the indexers, indexer-storage, and indexer journal client (and a fortiori point 3 above); because these three components use almost every other component, and aren't critical to the infrastructure.

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Migrated from T3229 (view on Phabricator)