[MM-71072] Fix broken Security Bulletin signup link on Security Updates docs page (#38928)

The Security Bulletin signup link pointed at
https://mattermost.com/security-updates/#sign-up. That page now
301-redirects to https://docs.mattermost.com/security-guide/security-updates,
so readers were bounced back to the page they started on and never reached a
signup form. Point the link at the Security Bulletin section of the
responsible disclosure page instead.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: mattermost-code <matty-code@mattermost.com>

docs/main/security-guide/security-updates.mdx

@@ -6,7 +6,7 @@ hide_table_of_contents: true
 
 This bulletin lists publicly disclosed security issues for Mattermost Server, Desktop, Mobile, and Plugins after a fix is available. Use the table to check whether an advisory affects your deployment and which release contains the fix. Filter by product, severity, issue ID, date, or affected version.
 
-To report a vulnerability, follow the [Responsible Disclosure Policy](https://mattermost.com/security-vulnerability-report/). Subscribe to fix-release notifications on the [Security Bulletin signup](https://mattermost.com/security-updates/#sign-up).
+To report a vulnerability, follow the [Responsible Disclosure Policy](https://mattermost.com/security-vulnerability-report/). Subscribe to fix-release notifications on the [Security Bulletin signup](https://mattermost.com/company/responsible-disclosure/#security-bulletin).
 
 <Note>
 Specific details on security updates are announced 30 days after the fix is available. Mattermost has a [mandatory upgrade policy](/administration-guide/upgrade/upgrading-mattermost-server) and provides updates for supported releases.