Clarify that server-ca is optional for mTLS ingest

[JannikBrand]

What

Clarify that the server-ca value returned in service bindings is optional for mTLS ingest into SAP Cloud Logging.

The ingest endpoints (ingest-mtls, ingest-otlp) present a server certificate that chains to a public root (Let's Encrypt ISRG Root X1). Any runtime whose system trust store already carries the public roots — which is the default for CF stacks, SapMachine, current Node.js versions — needs no additional pinning. Instructing consumers to pin server-ca as a sole trust anchor was misleading and became actively wrong when the ingestion CA was rotated (a customer whose app pinned the previous intermediate saw ingest break).

Changes

• Ingest from the Cloud Foundry Runtime — remove server-ca from the user-provided-service (cf cups) example and its jq extractor. Loggregator's HTTPS writer adds a UPS-supplied ca additively to the platform's trust pool; it is not required when the endpoint chains to a public root.
• Ingest via the JSON API Endpoint — drop --cacert server-ca.crt from the curl example (call it out as optional in a note).
• Ingest via the OpenTelemetry API Endpoint — annotate server-ca in the sample credentials JSON as optional and add a clarifying note. Also relaxed the wording "the required endpoint and credentials" → "the endpoint and credentials".

Context

• Companion changes in the two SAP-provided OTel helper libraries follow up:

  • Make server-ca optional in Cloud Logging credentials SAP/cf-java-logging-support#432
  • Make server-ca optional and use it as additional trust anchor SAP/opentelemetry-exporter-for-sap-cloud-logging-for-nodejs#47

  Both currently reject a service binding without server-ca in their validate(), and, for the ones that accept it, pin it as sole anchor instead of appending it. Those PRs make the field optional and treat it additively.