chore(cli): stop creating connector secrets for native connectors

[Jan-Kazlouski-elastic]

Closes #2990

Follow-up to #4524. Since 9.0 (#2989) the framework never reads connector secrets, but connectors connector create --native still stored the connector API key in a connector secret and wrote its id to api_key_secret_id. Nothing consumes that secret.

Changes:

• CLI no longer creates a connector secret; api_key_secret_id is written as null
• Remove ESManagementClient.create_connector_secret() (CLI was the only caller) and its test
• Remove test_get_index_or_alias, a copy of the secret test that never called get_index_or_alias (covered by test_extract_index_or_alias_*)
• CONNECTOR_PROTOCOL.md: note api_key_secret_id is not read by the framework since 9.0

Deliberately kept:

• native_connector_api_keys in BaseDataSource.features(): part of the ES ConnectorFeatures schema; removing it would make native connector docs be rewritten on every scheduler poll, because the key sets would never match
• api_key_secret_id field: still part of the ES Connector API and written by Kibana

Not for 8.19: 8.19 still reads connector secrets during native syncs.

Checklists

Pre-Review Checklist

• this PR does NOT contain credentials of any kind, such as API keys or username/passwords (double check config.yml.example)
• this PR has a meaningful title
• this PR links to all relevant github issues that it fixes or partially addresses
• Covered the changes with automated tests
• Tested the changes locally (make clean install autoformat lint test)
• Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)

Release Note

The connectors connector create --native CLI command no longer stores the connector API key in a connector secret.

[github-actions]

💚 Backport PR(s) successfully created

Status	Branch	Result
✅	9.5	#4580
✅	9.4	#4581

The backport PRs will be merged automatically after passing CI.