NIAD-2991: Use the NarrativeStatement/availabilityTime for filing comment ObservationStatement.issued

[adrianclay]

Why

In NIAD-2991 we were provided with an anonymised GP2GP investigation from production, where the availabilityTime for:

• Test Group Header
• Test Result
• Filing Coment

Were the values which seemed to be the correct value to use for issued.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users
• A corresponding change has been made to the Mapping Documentation repository: NIAD-2991: Update mapping behaviour of Observation.issued patient-switching-adaptors-mapping-documentation#52