Decommission DXR

Hello,

tldr: We are planning to turn off DXR.m.o at the end of the year, please be on the lookout for breakage.

DXR, the successful successor of MXR, served us well over the years. Over the last few years, in parallel, so many terrific improvements have been made to searchfox (Rust support, code coverage integration, etc) that the most current users of DXR are now bots.

Therefore, in order to have one tool for code search, we will turn off DXR Tuesday, December 29th.

To make sure we are not missing anything, we plan to turn off DXR for 12 hours Monday November 2nd and wait for breakage in our tools or feedback.

In parallel, to make sure we have parity, we have deployed a few changes:

We will then set up a DXR => Searchfox redirect.

Meta bug: https://bugzilla.mozilla.org/show_bug.cgi?id=byebye-dxr

Sylvestre