Deprecate xpack.telemetry.* instead of removing it (#51117)

While telemetry plugin has moved and its configurations no longer have
the xpack prefix, for the time being we should only warn when we
encounter the existing config names rather than blocking startup
entirely.

src/legacy/server/config/transform_deprecations.js

@@ -102,6 +102,10 @@ const deprecations = [
   rename('optimize.lazyHost', 'optimize.watchHost'),
   rename('optimize.lazyPrebuild', 'optimize.watchPrebuild'),
   rename('optimize.lazyProxyTimeout', 'optimize.watchProxyTimeout'),
+  rename('xpack.telemetry.enabled', 'telemetry.enabled'),
+  rename('xpack.telemetry.config', 'telemetry.config'),
+  rename('xpack.telemetry.banner', 'telemetry.banner'),
+  rename('xpack.telemetry.url', 'telemetry.url'),
   savedObjectsIndexCheckTimeout,
   rewriteBasePath,
   configPath,