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

app/diagnostics.go

@@ -585,18 +585,18 @@ func (a *App) trackPlugins() {
 				}
 				if state, ok := pluginStates[plugin.Manifest.Id]; ok && state.Enable {
 					totalEnabledCount += 1
-					if plugin.Manifest.Backend != nil {
+					if plugin.Manifest.HasServer() {
 						backendEnabledCount += 1
 					}
-					if plugin.Manifest.Webapp != nil {
+					if plugin.Manifest.HasWebapp() {
 						webappEnabledCount += 1
 					}
 				} else {
 					totalDisabledCount += 1
-					if plugin.Manifest.Backend != nil {
+					if plugin.Manifest.HasServer() {
 						backendDisabledCount += 1
 					}
-					if plugin.Manifest.Webapp != nil {
+					if plugin.Manifest.HasWebapp() {
 						webappDisabledCount += 1
 					}
 				}