docs(README): update codecov badge

[musicEnfanthen]

This PR updates the codecov badge in the README to not use the main-branch-specific badge.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.25%. Comparing base (42b9017) to head (486db54).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1459   +/-   ##
========================================
  Coverage    76.25%   76.25%           
========================================
  Files          244      244           
  Lines         3996     3996           
  Branches       608      608           
========================================
  Hits          3047     3047           
  Misses         929      929           
  Partials        20       20           

Flag	Coverage Δ
unittests	76.25% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 42b9017...486db54. Read the comment docs.

[masthom]

Bin icht sicher, was ich hier erntshaft begutachten kann...?

Falls es nur der Form halber ist, gerne... ;-)

[musicEnfanthen]

Ja, ist ein Test für die Build-Infrastruktur.

Danke!