Release notes for ARC 6.18.0
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This merge request is automatically cherrypicked from !1637 (merged) from source branch https://source.coderefinery.org/nordugrid/arc/-/tree/release_notes_6.18.0 orginally made by @maikenp .
The original MR went to branch: https://source.coderefinery.org/nordugrid/arc/-/tree/master
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