Finish Codecov migration

[ohmyfelix]

What changed

• switched the README coverage badge from Coveralls to Codecov
• removed the obsolete tests/.coveralls.yml file
• kept the existing reusable coverage workflow wiring because it already uses nette-tester-coverage-v2

Why

• finishes the remaining Coveralls-to-Codecov cleanup for this repository
• removes stale Coveralls-specific config that is no longer used

Refs contributte/contributte#72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@b4450ea). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #121   +/-   ##
=========================================
  Coverage          ?   20.00%           
=========================================
  Files             ?        3           
  Lines             ?       35           
  Branches          ?        0           
=========================================
  Hits              ?        7           
  Misses            ?       28           
  Partials          ?        0           

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