Issue 508: Enforce deprecation of observable:creationTime

[ajnelson-nist]

This Pull Request is part of the resolution of Issue #508 .

Coordination

• [x] Pull Request is against correct branch
• [x] Pull Request is in, or reverted to, Draft status before Solutions Approval vote has passed
• [x] CI passes in UCO feature branch against develop-2.0.0
• [x] CI passes in UCO current unstable-2.0.0 branch (86c0d61)
• [x] CI passes in CASE current unstable-2.0.0 branch tracking UCO's unstable as submodule (5a97e5c)
• [x] Impact on SHACL validation reviewed for CASE-Examples
• [x] Impact on SHACL validation remediated for CASE-Examples (N/A)
• [x] Impact on SHACL validation reviewed for casework.github.io
• [x] Impact on SHACL validation remediated for casework.github.io (N/A)
• [x] Milestone linked
• [x] Solutions Approval vote logged on corresponding Issue (once logged, can be taken out of Draft PR status)