Fixed GET /_cat returning a text/plain response.

[dblock]

Description

The GET /_cat API (https://opensearch.org/docs/latest/api-reference/cat/index/) is not correctly documented. It does not accept any parameters as far as I can see and returns text/plain.

This PR fixes the schema for /_cat.

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[github-actions]

Changes Analysis

Commit SHA: 7104061
Comparing To SHA: 6083bf5

API Changes

Summary

├─┬Paths
│ └─┬/_cat
│   └─┬GET
│     ├──[➖] parameters (12063:13)❌ 
│     ├──[➖] parameters (12055:13)❌ 
│     └─┬Responses
│       └─┬200
│         ├──[➕] content (23169:11)
│         └──[➖] content (23190:11)❌ 
└─┬Components
  ├──[➖] parameters (12055:7)❌ 
  ├──[➖] parameters (12063:7)❌ 
  └──[➖] schemas (38442:7)❌ 

Document Element	Total Changes	Breaking Changes
paths	4	3
components	3	3

• ❌ BREAKING Changes: 6 out of 7
• Removals: 6
• Additions: 1
• Breaking Removals: 6

Report

The full API changes report is available at: https://github.com/opensearch-project/opensearch-api-specification/actions/runs/9638357292/artifacts/1629858253

API Coverage

	Before	After	Δ
Covered (%)	483 (47.31 %)	483 (47.31 %)	0 (0 %)
Uncovered (%)	538 (52.69 %)	538 (52.69 %)	0 (0 %)
Unknown	24	24	0