Add dependabot config

[artemgavrilov]

Some actions in our CI workflows are outdated and produce warnings in logs. Dependabot will simplify new versions tracking and update.

Same for pg_tde: Percona-Lab/pg_tde#169

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.55%. Comparing base (3d84610) to head (b399f9e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #443   +/-   ##
=======================================
  Coverage   90.55%   90.55%           
=======================================
  Files           3        3           
  Lines        1271     1271           
  Branches       35       35           
=======================================
  Hits         1151     1151           
  Misses         98       98           
  Partials       22       22           

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