Align test data for fmuobs with what is possible to validate

[daniel-sol]

Including ert-doc.obs in an ert config file doesn't validate in ert as of January 2024.
Some enquiry done, and at least two things stand out:

• BLOCK_OBSERVATIONs were deprecated sometime in 2022
• Date fields now have to be in iso 8601 format (YYYY-MM-DD)
  The tests were not picking this up, because the OBS_CONFIG keyword was not used in testing with ert
  In my opinion the test data should validate in ert otherwise it is not a good dataset.
  Suggestion is to modify test data accordingly, but to leave the code otherwise as is, for backward compatibility.
  This should not cause problems since the changes made to ert are a narrowing down, not a widening up..