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.

Mana

2

Assignee

Jesse Hallam

QA Assignee

Lindy Isherwood

Reporter

Joram Wilander

Epic Link

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Fix versions

Mattermost Team

ABC

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