e2e tests: run against latest in PRs, but also trunk and 6.2 in trunk

[priethor]

What

Updates the CI e2e matrix to run against WP latest (6.9 as of now) in PRs, but also the minimum supported version (6.2) and trunk when committing to trunk.

Why

To get fast feedback loops in PRs against the most recent WP version, yet thoroughly test a wide range of WP versions in trunk.

[github-actions]

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Props priethor.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 9.37%. Comparing base (49fe41f) to head (fa14631).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##             trunk    #269   +/-   ##
=======================================
  Coverage     9.37%   9.37%           
  Complexity    4288    4288           
=======================================
  Files          266     266           
  Lines        33159   33159           
=======================================
  Hits          3108    3108           
  Misses       30051   30051           

Flag	Coverage Δ
e2e	21.83% <ø> (ø)
phpunit	6.08% <ø> (ø)

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