Change telemetry optIn to default to true (#50490)

* Change optIn to default to true

* Update snapshot

* Fix eslint errors

src/legacy/core_plugins/telemetry/index.ts

@@ -50,10 +50,8 @@ const telemetry = (kibana: any) => {
         allowChangingOptInStatus: Joi.boolean().default(true),
         optIn: Joi.when('allowChangingOptInStatus', {
           is: false,
-          then: Joi.valid(true).required(),
-          otherwise: Joi.boolean()
-            .allow(null)
-            .default(null),
+          then: Joi.valid(true).default(true),
+          otherwise: Joi.boolean().default(true),
         }),
 
         // `config` is used internally and not intended to be set

src/legacy/core_plugins/telemetry/public/components/__snapshots__/telemetry_form.test.js.snap

@@ -35,7 +35,7 @@ exports[`TelemetryForm renders as expected when allows to change optIn status 1`
         setting={
           Object {
             "ariaName": "Provide usage statistics",
-            "defVal": false,
+            "defVal": true,
             "description": <React.Fragment>
               <p>
                 Help us improve the Elastic Stack by providing usage statistics for basic features. We will not share this data outside of Elastic.

src/legacy/core_plugins/telemetry/public/components/telemetry_form.js

@@ -117,7 +117,7 @@ export class TelemetryForm extends Component {
                 type: 'boolean',
                 value: telemetryOptInProvider.getOptIn() || false,
                 description: this.renderDescription(),
-                defVal: false,
+                defVal: true,
                 ariaName: i18n.translate('telemetry.provideUsageStatisticsLabel', { defaultMessage: 'Provide usage statistics' })
               }}
               save={this.toggleOptIn}

src/legacy/core_plugins/telemetry/server/telemetry_config/get_telemetry_opt_in.ts

@@ -44,7 +44,7 @@ export const getTelemetryOptIn: GetTelemetryOptIn = ({
   }
 
   if (telemetrySavedObject === null || typeof telemetrySavedObject.enabled !== 'boolean') {
-    return null;
+    return configTelemetryOptIn;
   }
 
   const savedOptIn = telemetrySavedObject.enabled;