Removes wrong 404 status code for getAllSubmodelDescriptors (3.2 Port)

[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:
  V3.2_SSP-001.yaml
• Removed 404 Not Found from GET /submodel-descriptors in the Submodel Registry read profile:
  V3.2_SSP-002.yaml
• Applied the same change in the aggregated OpenAPI collection:
  V3.2.yaml

Documentation Updates:

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

Related Issues

See #597

Cherry picked 30bd9be and 0d21dcb from #617

IMPORTANT!

Merge #618 first

[BirgitBoss]

approved, only one thing; I did not find the 403 fix in the changelog
The changelog contains more changes than the PR, so probably several PRs covered in changelog?

[aaronzi]

@BirgitBoss I tried to create the PRs in a way that merging them after each other is possible without causing merge conflicts. But this means that more stuff is included than what the PR actually fixes (e.g. change log entries from previous PRs). This is why the description says merge xyz first. I can change that if it's wanted.

The 404 fix is documented in the changelog.