Anchor mysql version to 8.4

[afterdusk]

In #433, we updated our docker-compose.yaml to use a startup argument relevant to the then latest mysql version (8.4).

However this startup argument is deprecated in the current latest version of mysql (9.0). I have not had time to understand what changes need to be made to be compatible - for now, let's set the mysql version to 8.4 to fix CI and unblock local development. We will upgrade to a more recent version when we are ready.

I also drop the version attribute from the docker-compose.yaml, since it is obsolete and produces a warning when used.

[afterdusk]

Added one more commit to rename .cargo/config to .cargo/config.toml, to address the warning that runs on every cargo invocation:

warning: `akd/.cargo/config` is deprecated in favor of `config.toml`

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.92%. Comparing base (3ce5335) to head (de8d4ca).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #443      +/-   ##
==========================================
- Coverage   88.61%   87.92%   -0.70%     
==========================================
  Files          39       38       -1     
  Lines        9109     8165     -944     
==========================================
- Hits         8072     7179     -893     
+ Misses       1037      986      -51     

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