Document MariaDB support in README

[robertsilen]

Mention MariaDB alongside MySQL in setup, sandbox, and bin/build
instructions. Solidus uses the same mysql2 adapter for both
(export DB=mysql); this is documentation only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (57280f7) to head (acf741f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6485   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files         991      991           
  Lines       20832    20832           
=======================================
  Hits        18683    18683           
  Misses       2149     2149           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.