Rename substitution_list to substitutions

[eivindjahren]

Its not a list, and in any case not useful information.

• 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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.96%. Comparing base (ca44e00) to head (7d1c594).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ert/substitutions.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9026      +/-   ##
==========================================
- Coverage   90.98%   90.96%   -0.02%     
==========================================
  Files         352      352              
  Lines       21695    21695              
==========================================
- Hits        19739    19735       -4     
- Misses       1956     1960       +4     

Flag	Coverage Δ
cli-tests	39.08% <87.87%> (-0.02%)	⬇️
gui-tests	72.73% <90.90%> (+0.02%)	⬆️
performance-tests	49.58% <81.81%> (-0.01%)	⬇️
unit-tests	79.69% <93.93%> (ø)

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[berland]

🧹 👍🏻