[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>

scripts/config_generator/main_test.go

@@ -25,7 +25,6 @@ func TestDefaultsGenerator(t *testing.T) {
 	b, err := ioutil.ReadFile(tmpFile.Name())
 	require.NoError(t, err)
 	require.NoError(t, json.Unmarshal(b, &config))
-	require.True(t, *config.ServiceSettings.DisableLegacyMFA)
 	require.Equal(t, *config.SqlSettings.AtRestEncryptKey, "")
 	require.Equal(t, *config.FileSettings.PublicLinkSalt, "")