chore: add downloads in README, security policy and update ci actions (#401)

* add security policy

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* update deprecated actions

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add comment about licenses for new dependencies

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add pypi downloads badge

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add citation file

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

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Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>