Enable strict merge on Mergify

[kalikiana]

• "strict merge" means that open PRs are checked again whenever the target branch has changed. Affected branches can be rebase automatically.
• Behavior for merge-fast will not change.
• The new label no-automatic-rebase can be used to retain the current behavior (i.e. your branch is not touched and only merged as-is if possible).

See https://docs.mergify.io/merge-action.html#strict-merge

[kalikiana]

One idea @perlpunk brought up: we could introduce a separate label for PRs that you don't want to be rebased or re-run, like no-automatic-rebase.

[okurz]

we can try :)

[codecov]

Codecov Report

Merging #3452 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3452      +/-   ##
==========================================
+ Coverage   95.52%   95.59%   +0.06%     
==========================================
  Files         366      366              
  Lines       31556    31557       +1     
==========================================
+ Hits        30144    30166      +22     
+ Misses       1412     1391      -21     

Impacted Files	Coverage Δ
lib/OpenQA/Parser/Format/XUnit.pm	98.33% <100.00%> (+0.02%)	⬆️
t/05-scheduler-full.t	97.94% <0.00%> (+0.51%)	⬆️
t/lib/OpenQA/SeleniumTest.pm	89.07% <0.00%> (+1.09%)	⬆️
t/lib/OpenQA/Test/Utils.pm	68.94% <0.00%> (+3.70%)	⬆️
t/43-scheduling-and-worker-scalability.t	91.86% <0.00%> (+4.06%)	⬆️

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