Drop about:tbupdate

With the new design from #41333 (closed), we removed the link to "about:tbupdate" in the top right corner, underneath the version number. This was the only link to the "about:tbupdate" page, so it no longer has any external links (not even in "about:about").

Note: when there is an update, we show a link to the blog post in "about:tor". This was also true for the old "about:tor" design.

Given that "about:tbupdate" simply prints out the top of ChangeLog.txt, I think we can probably eventually retire the page, and redirect users to check online, or the local ChangeLog.txt file if they are interested.

Any thoughts, or any insights into why users would want to keep the page?

/cc @richard @pierov @donuts