fix: copy for literal search toast to make it applicable to new users

Created by: attfarhan

It is difficult to only show the new literal search popup notification for "old" users who have been using sourcegraph while the default was regexp search, since we can't currently detect upgrades.

Instead of showing the popup only to new users, @slimsag and I discussed that it might be best to update the copy of the popup, to make it similar to a guided onboarding modal. Now, the toast simply describes the regexp toggle, not mentioning "new". It is useful for new users to know they can do regexp search anyways, so we wouldn't have to remove this in a future release either.

Addresses the second part of https://github.com/sourcegraph/sourcegraph/issues/6028