Wrong worksheet name in migration yaml files.

Problem/Motivation

In #3567171: Update XLS file the downloaded Excel file was updated to a new one from Bpost. I never saw the old one, but in the migration yaml files you can see this:
worksheet: "Plaatsnamen op postcode"
The current Excel file (https://www.bpost.be/sites/default/files/zipcodes/zipcodes_num_nl_2025.xls) does not have a worksheet named Plaatsnamen op postcode but does have one named Plaatsnamen per postcode so this might have changed with the new file.

This causes this issue when importing:

Got 'PhpOffice\PhpSpreadsheet\Exception', message 'You tried to set a sheet active by the out of bounds index: 0. The actual number of sheets is 0.'.

And nothing gets imported.

We need to update the worksheet name in the migration files to Plaatsnamen op postcode

Steps to reproduce

Install the module or run the migration with drush migrate-import belgian_postal_code_main_municipalities