Add dependabot config

[solnic]

To actually test it out we need to:

1. Merge this in
2. Wait for a new dependabot run and verify if it succeeded
3. Downgrade some of the deps to something old with vulnerabilities and see if Dependabot will create a PR for bumping it

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.33%. Comparing base (a035f96) to head (7498e07).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2551   +/-   ##
=======================================
  Coverage   69.33%   69.33%           
=======================================
  Files         126      126           
  Lines        4728     4729    +1     
=======================================
+ Hits         3278     3279    +1     
  Misses       1450     1450           

Components	Coverage Δ
sentry-ruby	60.12% <ø> (ø)
sentry-rails	97.48% <ø> (ø)
sentry-sidekiq	95.52% <ø> (ø)
sentry-resque	92.64% <ø> (+0.10%)	⬆️
sentry-delayed_job	95.65% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

see 1 file with indirect coverage changes