[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>

config/client_test.go

@@ -111,33 +111,6 @@ func TestGetClientConfig(t *testing.T) {
 				"EnforceMultifactorAuthentication": "true",
 			},
 		},
-		{
-			"experimental channel organization enabled",
-			&model.Config{
-				ServiceSettings: model.ServiceSettings{
-					ExperimentalChannelOrganization: model.NewBool(true),
-				},
-			},
-			"tag1",
-			nil,
-			map[string]string{
-				"ExperimentalChannelOrganization": "true",
-			},
-		},
-		{
-			"experimental channel organization disabled, but experimental group unread channels on",
-			&model.Config{
-				ServiceSettings: model.ServiceSettings{
-					ExperimentalChannelOrganization: model.NewBool(false),
-					ExperimentalGroupUnreadChannels: model.NewString(model.GroupUnreadChannelsDefaultOn),
-				},
-			},
-			"tag1",
-			nil,
-			map[string]string{
-				"ExperimentalChannelOrganization": "true",
-			},
-		},
 		{
 			"default marketplace",
 			&model.Config{