Update test postgres and mariadb versions

[albertyw]

This updates the versions of databases that are used for testing django-silk

• Update postgres to the oldest and newest supported versions of postgres
• Update mariadb to the supported LTS versions (except for 10.5)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.35%. Comparing base (483a08d) to head (05879a1).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #750      +/-   ##
==========================================
+ Coverage   87.21%   87.35%   +0.14%     
==========================================
  Files          52       52              
  Lines        2135     2135              
==========================================
+ Hits         1862     1865       +3     
+ Misses        273      270       -3     

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