command/output: Ignore warnings in JSON mode

[radeksimko]

Fixes #38512
Closes #38513

Ideally we would just print out the diagnostics in the JSON mode, like we do in other JSON formatted output from other commands, but as @SarahFrench pointed out, this cannot be done easily (without breaking changes) since the format does not currently support versioning and doesn't allow for easy expansion:

{
	"foo":{
		"sensitive":false,
		"type":"string",
		"value":"bar"
	}
}

If we were to break things in the future and introduce the diagnostics, the likely format would be something like this:

{
  "format_version": "1.0",
  "outputs": {
    "greeting": {
        "sensitive": false,
        "type": "string",
        "value": "hello world"
    }
  },
  "diagnostics": []
}

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[SarahFrench]

Thanks for fixing this! The day we can get JSON output (more) standardised will be a good day 🌥️