Mention now mandatory `expires_at` parameter in the documentation

What does this MR do and why?

Improves documentation related to programmatical token creation.

Expiration date is mandatory for tokens since GitLab 16.0 but this fact is not reflected yet in the documentation: https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html#create-a-personal-access-token-programmatically

It results in the examples from the documentation to be not working.

This MR makes examples working again.

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