Add flag to keep legacy extensions system enabled

Created by: philipp-spiess

As a result from this thread, we'll add another flag that can be used to turn on legacy extensions for instances past 4.0: https://sourcegraph.slack.com/archives/C03P1V3J92P/p1658488293653899

Ideally we can hid this somehow so that we keep usage at a minimum (ideally we know everyone who uses this and can work with the customers on a reasonable upgrade path for them).