doc: ci/yaml expire_in: the value is in seconds

What does this MR do?

Our sysadmin configured default expire to 45 with the intention to set expiry to 45 days.

After that jobs started to fail because artifacts were already expired by the time next job ran.

This updates documentation, for this to be clearer.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated