Refactor storage migrations

[oyvindeide]

Issue
Resolves #7378

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• 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 tests pass locally (after every commit!)

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 97.29730% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.15%. Comparing base (b090537) to head (4b447b2).

Files	Patch %	Lines
src/ert/storage/migration/to5.py	93.10%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7570       +/-   ##
===========================================
+ Coverage   34.92%   85.15%   +50.23%     
===========================================
  Files         388      385        -3     
  Lines       23323    23295       -28     
  Branches      893      876       -17     
===========================================
+ Hits         8146    19838    +11692     
+ Misses      15070     3343    -11727     
- Partials      107      114        +7     

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[pinkwah]

LGTM with nitpick