Make cardano-db backend v2 as default

[turmelclem]

Content

The default version used for the client-CLI command cardano-db is now v2.

This PR includes...

• make cardano-db with version 2 as default ;
• update documentation with updated cardano-db v2 examples ;
• adapt Aggregator stress test to use v2 endpoints, and signed entity type CardanoDatabase instead of ImmutableFilesFull
• update the CI (test-client.yml) to :
  • remove optional --backend v2 from cardano-db calls ;
  • add --backend v1 when no version was specified.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)
  • Add ADR blog post or Dev ADR entry (if relevant)
  • No new TODOs introduced

Closes #2701

[github-actions]

Test Results

    4 files  ±0    164 suites  ±0   23m 47s ⏱️ +12s
2 182 tests ±0  2 182 ✅ ±0  0 💤 ±0  0 ❌ ±0 
6 709 runs  ±0  6 709 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 45ff226. ± Comparison against base commit 67b18cd.

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍