Replace deprecated polars count with len

[larsevj]

Get following warning when running tests:

tests/ert/unit_tests/gui/simulation/test_run_path_dialog.py: 6 warnings
  /data/komodo_dev/ert/src/ert/gui/ertwidgets/models/ertsummary.py:40: DeprecationWarning: `GroupBy.count` is deprecated. It has been renamed to `len`.
    counts.extend(df.group_by("observation_key").count().to_dicts())  #  type: ignore

• 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 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

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

[xjules]

Nice one! 🚀