[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/post.go

@@ -213,7 +213,7 @@ func (a *App) CreatePost(c *request.Context, post *model.Post, channel *model.Ch
 		post.AddProp("from_bot", "true")
 	}
 
-	if a.Srv().License() != nil && *a.Config().TeamSettings.ExperimentalTownSquareIsReadOnly &&
+	if a.Srv().License() != nil &&
 		!post.IsSystemMessage() &&
 		channel.Name == model.DefaultChannelName &&
 		!a.RolesGrantPermission(user.GetRoles(), model.PermissionManageSystem.Id) {