logstash Doc: Add clarification about API key format for Logstash

Release notes

[rn:skip]

What does this PR do?

This PR clarifies the API key format required for Logstash in the following docs:

docs/reference/secure-connection.md
docs/reference/connecting-to-cloud.md
docs/reference/using-logstash-with-elastic-integrations.md

Related issues

Closes #17687

Changes

(All links are to the PR preview)

Added clarification in the Security to Elastic Cloud Serverless and Security to hosted Elasticsearch Service admonitions under Elasticsearch security on by default
Added a clarifying note in Sending data to Elastic Cloud (hosted Elasticsearch Service)
Added a tip and a screenshot in Grant access using API keys
Updated the annotations in the code examples under Create an API key and Extend Integrations with Logstash to make them more explicit with respect to the API key format and how it is constructed
Added a link to the Grant access using API keys doc on the Logstash page

Previews

https://docs-v3-preview.elastic.dev/elastic/logstash/pull/17688/reference https://docs-v3-preview.elastic.dev/elastic/logstash/pull/17688/reference/secure-connection https://docs-v3-preview.elastic.dev/elastic/logstash/pull/17688/reference/connecting-to-cloud https://docs-v3-preview.elastic.dev/elastic/logstash/pull/17688/reference/using-logstash-with-elastic-integrations