Replace interface towards eclrun and flow

[berland]

Issue
Resolves #8925

Approach

• "ecl_config"-concept is eradicated, this was the legacy way of running Eclipse prior to eclrun from SLB.
• The eclrun wrapper we had was called EclRun. This is renamed to RunReservoirSimulator and handles both Eclipse and Flow.
• Assumes there is a script/binary in $PATH called flowrun that has quite similar CLI as eclrun.

(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 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

[berland]

Superseded by #9108