Fix deprecation warning from polars

[larsevj]

Fixes the warning everest_storage.py:507: DeprecationWarning: The argument columnsforDataFrame.pivotis deprecated. It has been renamed toon``

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[yngve-sk]

LGTM ⛳️

[codspeed-hq]

CodSpeed Performance Report

Merging #10300 will not alter performance

_{Comparing larsevj:fix_deprecation_warning_polars (15f9ed9) with main (ec0a372)}

Summary

✅ 25 untouched benchmarks