Revert SNAKE_OIL_PARAM_{param} to single param-group

[eilskra]

Issue
Resolves #14016

Approach
Reverted from multiple parameter groups back to a single one. "Issue" seems to stem from how the seeding/sampling is done (Link). I guess it since its just that it was a different seed/prior sampling, so it just looked different. Unsure if this is something we care about and/or if this is something that should be reverted.

Pre-revert:

seed = f(global_seed, group_name, param_name)  
       ├─ global_seed : RANDOM_SEED
       ├─ group_name  : SNAKE_OIL_PARAM_OP1
       └─ param_name  OP1_PERSISTENCE 

Post-revert

seed = f(global_seed, group_name, param_name)  
       ├─ global_seed : RANDOM_SEED
       ├─ group_name  : SNAKE_OIL_PARAM <- Now all share the same group again
       └─ param_name  : e.g. OP1_PERSISTENCE 

(I would like to thank Copilot for the illustration)

Visual example
Pre-revert:

Post-revert:

• 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 screenshots are changed: Review screenshot-PR in ert-testdata,
  merge screenshot-PR in ert-testdata before merging this PR.
• 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: Add backport label to latest release (format: 'backport release-branch-name')

[ertomatic]

Screenshots differ from baselines. A baseline update PR has been prepared: equinor/ert-testdata#58

[berland]

👍🏻