Rename "DGIS modified" in DB to "MWC modified"

[MichaelAkvo]

While working on #4809 and testing the changes, it was discovered that the ProjectEditorValidationSet in production is called "DGIS modified" in the DB. In the frontend it's hardcoded as "MWC modified".

For consistency, the name should be updated.

TODO

• Update name in production (either migration or manual action)
• Update the fixture: akvo-rsr/akvo/rsr/fixtures/project_validations.yaml