[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>

web/webhook_test.go

@@ -129,32 +129,6 @@ func TestIncomingWebhook(t *testing.T) {
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 	})
 
-	t.Run("WebhookExperimentalReadOnly", func(t *testing.T) {
-		th.App.Srv().SetLicense(model.NewTestLicense())
-		th.App.UpdateConfig(func(cfg *model.Config) { *cfg.TeamSettings.ExperimentalTownSquareIsReadOnly = true })
-
-		// Read only default channel should fail.
-		resp, err := http.Post(url, "application/json", strings.NewReader(fmt.Sprintf("{\"text\":\"this is a test\", \"channel\":\"%s\"}", model.DefaultChannelName)))
-		require.NoError(t, err)
-		assert.True(t, resp.StatusCode != http.StatusOK)
-
-		// None-default channel should still work.
-		resp, err = http.Post(url, "application/json", strings.NewReader(fmt.Sprintf("{\"text\":\"this is a test\", \"channel\":\"%s\"}", th.BasicChannel.Name)))
-		require.NoError(t, err)
-		assert.True(t, resp.StatusCode == http.StatusOK)
-
-		// System-Admin Owned Hook
-		adminHook, appErr := th.App.CreateIncomingWebhookForChannel(th.SystemAdminUser.Id, th.BasicChannel, &model.IncomingWebhook{ChannelId: th.BasicChannel.Id})
-		require.Nil(t, appErr)
-		adminURL := apiClient.URL + "/hooks/" + adminHook.Id
-
-		resp, err = http.Post(adminURL, "application/json", strings.NewReader(fmt.Sprintf("{\"text\":\"this is a test\", \"channel\":\"%s\"}", model.DefaultChannelName)))
-		require.NoError(t, err)
-		assert.Equal(t, http.StatusOK, resp.StatusCode)
-
-		th.App.UpdateConfig(func(cfg *model.Config) { *cfg.TeamSettings.ExperimentalTownSquareIsReadOnly = false })
-	})
-
 	t.Run("WebhookAttachments", func(t *testing.T) {
 		attachmentPayload := `{
 	       "text": "this is a test",