Replace `db:migrate` with `db:prepare`

[MrSerth]

db:prepare was introduced with Rails 6 and performs the necessary steps in a multi-database Rails app to bring the database to the required state. If non-existent, a new database is created and the schema is loaded; otherwise, migrations are performed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.40%. Comparing base (4eab086) to head (0b20fcb). Report is 33 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2524 +/- ## ======================================= Coverage 69.40% 69.40% ======================================= Files 199 199 Lines 6347 6347 ======================================= Hits 4405 4405 Misses 1942 1942 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.