Deprecate and remove `ExtraUpdateAt` field from channel model

Description

It's no longer being used and some related queries were causing performance issues.

See https://github.com/mattermost/mattermost-server/pull/8415

QA Test Steps

No testing required.

Status

Mana

2

Assignee

Jesse Hallam

Reporter

Joram Wilander

Epic Link

None

Fix versions

Mattermost Team

ABC

Sprint

None

Labels

None

GitHub Issue

None

QA Assignee

Lindy Isherwood