Remove wrong 404 status code for getAllSubmodelDescriptors

[aaronzi]

This pull request aligns the Submodel Registry response contract for GET /submodel-descriptors with collection semantics.
When no submodel descriptors exist, the endpoint returns 200 OK with an empty result set, so documenting 404 Not Found for this operation was misleading. This PR removes that ambiguity and updates the changelog accordingly.

API Specification Fixes:

• Removed 404 Not Found from GET /submodel-descriptors in the Submodel Registry full profile:
  V3.1_SSP-001.yaml
• Removed 404 Not Found from GET /submodel-descriptors in the Submodel Registry read profile:
  V3.1_SSP-002.yaml
• Applied the same change in the aggregated OpenAPI collection:
  V3.1.yaml

Documentation Updates:

• Added a 3.1.3 minor-change entry describing this fix and referencing issue 597:
  changelog.adoc

Related Issues

See #597

IMPORTANT!

Merge #616 first