[input] unifomize date format for created_at

input/grid5000/sites/grenoble/clusters/dahu/dahu.yaml

input/grid5000/sites/grenoble/clusters/yeti/yeti.yaml

input/grid5000/sites/lyon/clusters/hercule/hercule.yaml

input/grid5000/sites/lyon/clusters/nova/nova.yaml

input/grid5000/sites/lyon/clusters/orion/orion.yaml

...t/grid5000/sites/lyon/clusters/sagittaire/sagittaire.yaml

input/grid5000/sites/lyon/clusters/taurus/taurus.yaml

input/grid5000/sites/nancy/clusters/graphique/graphique.yaml

input/grid5000/sites/nantes/clusters/ecotype/ecotype.yaml