Add Dheeraj to the speakers list

Why is this change being made?

This adds Dheeraj Joshi (@djadmin) to the Speakers Bureau page.

I like to speak at Conferences / Meetups mainly about web security stuff.

I following the process listed at https://about.gitlab.com/handbook/marketing/developer-relations/developer-evangelism/speakers-bureau/#join-the-speakers-bureau.

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