Update Header with application/json

[safakkizkin]

Summary

Assigned content type application/json for NotFound (404), to stay true to the document.

Ticket Link

Fixes https://github.com/mattermost/mattermost-server/issues/20878 JIRA: https://mattermost.atlassian.net/browse/MM-46605

Release Note

NONE

[mm-cloud-bot]

@safakkizkin: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[mattermod]

Hello @safakkizkin,

Thanks for your pull request! A Core Committer will review your pull request soon. For code contributions, you can learn more about the review process here.

[agnivade]

@noxer - mind taking a look ? :)

[noxer]

Hi @safakkizkin and thank you for your contribution. I'd like to request a small change and then this should be good to go.

[safakkizkin]

Hi @noxer, yes you are totaly right!

[mattermod]

Successfully triggered e2e testing! https://git.internal.mattermost.com/qa/cypress-ui-automation/-/pipelines/243862

[noxer]

ping @isacikgoz

edit: Sorry, Github showed the first comment as not sent.

[safakkizkin]

rebased to be sure everything is great

[noxer]

/e2e-test

[mattermod]

Successfully triggered e2e testing! https://git.internal.mattermost.com/qa/cypress-ui-automation/-/pipelines/245490

[noxer]

Merged, thank you!