Update service protocol startup message to not reference Observatory

[bkonyi]

As we're encouraging users to make use of non-Observatory debugging tools like DevTools we should update the message the VM service outputs at startup to not reference Observatory. A possible alternative would be:

Dart VM service listening on http://127.0.0.1:8181/6iSIXM0fOBk=/

Overall, this change is relatively low priority as it will require changes throughout many of our tools which are scraping stdout for the current message to get the URL. Once #38418 is resolved and tools start migrating to use this functionality to discover the VM service, this change should be much simpler to make.

cc @devoncarew

[bkonyi]

Now that we've published new versions of the Dart Intellij plugin, the VM service message change has relanded in b036052. Assuming it sticks, this change will be visible starting in the 2.17 stable release.