doc(README): we don't use mongodb anymore

[Psycojoker]

There are still a lot of other MongoDB references still present that we probably want to get ride off but they are in the documentation and I think this need a whole rewrite?

❯ rg -i mongo
CHANGELOG.rst
135:- Decommissioned the support for RocksDB. The only supported storage engine is now MongoDB.

deployment/samples/docker-monitoring/sample-config.yml
20:mongodb:
21:  uri: "mongodb://mongodb:27017"

docs/figures/architecture-stack.pdf
679:(MongoDB) Tj

docs/figures/PyAleph architecture.graphml
487:						<y:Label.Text>MongoDB</y:Label.Text>

docs/guides/install.rst
132:Download the Docker Compose file that defines how to run PyAleph, MongoDB and IPFS together.
176:    * - nfuser_mongodb_1
177:      - docker-entrypoint.sh mongo ...
212:Check PyAleph data via MongoDB
215:MongoDB message counts
219:    docker exec -ti --user mongodb debian_mongodb_1 bash
220:    $ mongo

tests/api/test_list_messages.py
29:    like MongoDB object IDs.

src/aleph/db/accessors/chains.py
112:    #       replace it with the range/multirange operations of PostgreSQL 14+ once the MongoDB

deployment/docker-build/README.md
25:To run the local dev environment, you will need to set the P2P daemon, IPFS and MongoDB hosts to `127.0.0.1`.
44:This will instantiate the services for MongoDB, IPFS and the P2P daemon.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.65%. Comparing base (a65131f) to head (efce171).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #567      +/-   ##
==========================================
+ Coverage   94.63%   94.65%   +0.01%     
==========================================
  Files          89       89              
  Lines        4749     4748       -1     
  Branches      650      650              
==========================================
  Hits         4494     4494              
+ Misses        233      232       -1     
  Partials       22       22              

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