Update notification banner with new webinar details (mattermost#6236)

Author: emdecr

source/templates/header.html

@@ -4,7 +4,7 @@
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-		    <div><a href="https://mattermost.com/webinar/streamline-incident-response-with-servicenow/">Learn how to streamline incident response with Mattermost & ServiceNow on February 16th »</a></div>
+		    <div><a href="https://mattermost.com/webinar/release-management-with-gitlab/">Learn how to streamline and standardize your release processes with Mattermost and GitLab on March 2nd »</a></div>
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