[MM-36444] Manage config deprecations for 6.0 (#18035)

* removing deprecated stuff

* removing deprecations and marking others for deprecations

* removing some of the configs

* partially remove ExperimentalChannelOrganization

* fixing merge issues

* moving default permissions to defaultroles function

* removing experimentaltownsquare

* removing old tests and remove legacysidebar config

* fixing test

* removing log

* removing unused variables

* MM-36444: Fix for renamed functions.

* MM-36444: go mod tidy and go mod vendor.

* MM-36444: Vendored go modules.

* MM-36444: Removes UseExperimentalGossip config field from model.

* Fix vendors

* fixing test

Co-authored-by: Benjamin Cooke <benjamincooke@Benjamins-MacBook-Pro.local>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Martin Kraft <martin@upspin.org>
Co-authored-by: Claudio Costa <cstcld91@gmail.com>

app/webhook.go

@@ -777,7 +777,7 @@ func (a *App) HandleIncomingWebhook(c *request.Context, hookID string, req *mode
 	}
 	user = result.Data.(*model.User)
 
-	if a.Srv().License() != nil && *a.Config().TeamSettings.ExperimentalTownSquareIsReadOnly &&
+	if a.Srv().License() != nil &&
 		channel.Name == model.DefaultChannelName && !a.RolesGrantPermission(user.GetRoles(), model.PermissionManageSystem.Id) {
 		return model.NewAppError("HandleIncomingWebhook", "api.post.create_post.town_square_read_only", nil, "", http.StatusForbidden)
 	}