migrate to mongoose 8

[AlvaroVega]

Due to https://github.com/telefonicaid/iotagent-node-lib/security/dependabot/14

(maybe mongoose 5 will receive a backport: Automattic/mongoose#15078 (comment))

There is some gap to migrate from mongoose 5 to 8

https://mongoosejs.com/docs/migrating_to_6.html
https://mongoosejs.com/docs/migrating_to_8.html

#1673