Adding last updated to summary endpoint [ENG-1879]

[corbinSanders]

Purpose

The front end needs the datetime of when institutional metrics were last updated

Changes

Adding field to serializer. Getting the last updated datetime. Updating tests

QA Notes

• Does this change require a data migration? If so, what data will we migrate? No
• What is the level of risk? minimal
  • Any permissions code touched? No
  • Is this an additive or subtractive change, other? Additive
• How can QA verify? (Through UI, API, AdminApp or AdminAdminApp?) API
  • If verifying through API, what's the new version? Please include the endpoints in PR notes or Dev docs. No new version. Use v2/institutions/<institution_id>/metrics/summary/
• What features or workflows might this change impact? Institutional dashboard
• How will this impact performance? Shouldn't -->

Documentation

N/A

Side Effects

N/A

Ticket

https://openscience.atlassian.net/browse/ENG-1879