Support server, deprecate backend in plugin manifest (#9127)

* Support `server`, deprecate `backend` in plugin manifest

This lets us converge on the use of the term `server` everywhere instead
of sometimes `backend` and sometimes `server`. We're still using
`webapp` and will eventually support `mobile` as well.

The plan is actually to rip out these deprecations as part of releasing
5.2, but I want to coordinate the extra additional breakage at the same
time, so for now this is a backwards compatible change.

* fix failing tests

plugin/environment.go

@@ -192,7 +192,7 @@ func (env *Environment) Activate(id string) (reterr error) {
 		}
 	}
 
-	if pluginInfo.Manifest.Backend != nil {
+	if pluginInfo.Manifest.HasServer() {
 		supervisor, err := newSupervisor(pluginInfo, env.logger, env.newAPIImpl(pluginInfo.Manifest))
 		if err != nil {
 			return errors.Wrapf(err, "unable to start plugin: %v", id)