Disable gUM: media.devices.enumerate.legacy.enabled

I have actually been following 1528042 for four years

• https://groups.google.com/a/mozilla.org/g/dev-platform/c/AJp9KF5Ml3w/m/ISyYK9lBAQAJ
• pref gets flipped upstream at 1843434

notes

• TZP tests this without waiting for user actions (or initiating webrtc etc)
• RFP already mitigates enumeration by returning 1 audioinput and 1 videoinput (in that order)
• the legacy pref will not stay around forever

We should investigate that RFP holds when the pref set to false

cc: @pierov @richard ... kicking down the road to ESR128 (edit: unless they backport the pref flip to ESR115, but I've already done most of the investigation anyway)