MM-41988 Updated links to legacy domain about.mm.com (#19552)

* Updated links to legacy domain about.mm.com

* Legacy link updates

* Reverting - only need to change en.json

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* Reverting - only need to change en.json

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* Reverting - only need to change en.json

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* Reverting - only need to change en.json

* Reverting - only need to change en.json

* Reverting - only need to change en.json

* Reverting - only need to change en.json

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* Reverting - only need to change en.json

* Reverting - only need to change en.json

* about.mm.com URL updates

* mattermost.org URL updates

* forum.mm.org URL update

* Update .github/ISSUE_TEMPLATE.md

* Update .github/ISSUE_TEMPLATE.md

* Un-deleted language files

* Update README.md

* Update tests/test-config.json

* fix some test due to url updating (#19787)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Harrison Healey <harrisonmhealey@gmail.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

SECURITY.md

@@ -17,7 +17,7 @@ Security updates
 
 Mattermost has a mandatory upgrade policy, and updates are only provided for the latest 3 releases and the current Extended Support Release (ESR). Critical updates are delivered as dot releases. Details on security updates are announced 30 days after the availability of the update.
 
-For more details about the security content of past releases, see the [Security Updates](https://mattermost.com/security-updates/) page on the Mattermost website. For timely notifications about new security updates, subscribe to the [Security Bulletins Mailing List](https://about.mattermost.com/security-bulletin).
+For more details about the security content of past releases, see the [Security Updates](https://mattermost.com/security-updates/) page on the Mattermost website. For timely notifications about new security updates, subscribe to the [Security Bulletins Mailing List](https://mattermost.com/security-updates/#sign-up).
 
 Contributing to this policy
 ---------------------------