[FIX] hr_skills: fix resume end date wrong selection

before this commit, in resume we are able to put end date less than start date
which is not possible in real life.
make change in constraint to check selected end date is greater than start date.
make changes in demo data of end date which are according to new changes.

task-3397726

closes odoo/odoo#127351

Signed-off-by: Sofie Gvaladze (sgv) <sgv@odoo.com>