Update node to 20 on cloud fixity google functions

[hackartisan]

Google will stop providing support for the node version we're currently using on January 1, 2025. And the version we're using is EOL.

see:

• https://cloud.google.com/functions/docs/runtime-support#support_schedule
• https://endoflife.date/nodejs

We should update to 20, which we'll be doing for everything by the end of the calendar year. Example PR: https://github.com/pulibrary/figgy/commit/8f68698298f6ce96ede8955abecb87350ef0c926

Note: Make sure test suite passes before making the change, and again after.