Merge branch 'feature/add-istl-patch-2.6' into 'master'

Timo Kochrequested to merge
cherry-pick-4c6adfa4 into releases/3.0

[patches] Update istl patch to 2.6

See merge request !1454 (merged)

(cherry picked from commit 4c6adfa4)

d7803af7 [patches] Update istl patch to 2.6

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