[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>

api4/post.go

@@ -725,7 +725,6 @@ func saveIsPinnedPost(c *Context, w http.ResponseWriter, isPinned bool) {
 	}
 
 	if c.App.Srv().License() != nil &&
-		*c.App.Config().TeamSettings.ExperimentalTownSquareIsReadOnly &&
 		channel.Name == model.DefaultChannelName &&
 		!c.App.RolesGrantPermission(user.GetRoles(), model.PermissionManageSystem.Id) {
 		c.Err = model.NewAppError("saveIsPinnedPost", "api.post.save_is_pinned_post.town_square_read_only", nil, "", http.StatusForbidden)