docs: Update UPDATING.md

[cccs-joel]

SUMMARY

Ref: #17360
As requested, a new entry has been to the downtime section.

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[villebro]

@cccs-joel I would recommend closing this PR and incorporating these changes into #17360 so all related changes are in one PR.

[cccs-joel]

@cccs-joel I would recommend closing this PR and incorporating these changes into #17360 so all related changes are in one PR.

Ok, done, thanks!