[release-1.11] Temporarily pin Flatcar version to v1.26.10

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #4321

/assign willie-yao

NONE

[CecileRobertMichon]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3aa2a20ad5041da76035f75941e5ebe534eb43b4

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4c6b477) 56.71% compared to head (b2d0b51) 56.70%.
Report is 1 commits behind head on release-1.11.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.11    #4363      +/-   ##
================================================
- Coverage         56.71%   56.70%   -0.02%     
================================================
  Files               187      187              
  Lines             19222    19222              
================================================
- Hits              10901    10899       -2     
- Misses             7691     7693       +2     
  Partials            630      630              

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