MM-52532: Fix golangci warnings (#23709)

https://mattermost.atlassian.net/browse/MM-52532

- Replace golint with revive
- Add makezero linter
- Fix all the required linter failures

Some issues in enterprise and public modules
are yet to be fixed. We send this to expediate things.

server/public/model/notify_admin.go

@@ -26,13 +26,13 @@ const (
 	PluginFeature                      = MattermostFeature("mattermost.feature.plugin")
 )
 
-var validSKUs map[string]struct{} = map[string]struct{}{
+var validSKUs = map[string]struct{}{
 	LicenseShortSkuProfessional: {},
 	LicenseShortSkuEnterprise:   {},
 }
 
 // These are the features a non admin would typically ping an admin about
-var paidFeatures map[MattermostFeature]struct{} = map[MattermostFeature]struct{}{
+var paidFeatures = map[MattermostFeature]struct{}{
 	PaidFeatureGuestAccounts:           {},
 	PaidFeatureCustomUsergroups:        {},
 	PaidFeatureCreateMultipleTeams:     {},