[Visualization-utils] Fix getTimezone default value (#220658)

## Summary

fix #220644

This PR fixes the mentioned issue by defaulting any non-valid IANA
timezone to the current browser client timezone.

I'm also calling `moment.tz.guess(true)` with the true value because if
not moment will cache the first timezone guessed and will not be updated
if the customer change it manually.
>By default Moment Timezone caches the detected timezone. This means
that subsequent calls to moment.tz.guess() will always return the same
value.
You can call moment.tz.guess() with an optional boolean argument
"ignoreCache". If set to true, the cache will be ignored and overwritten
with the new value.


## Release note 

The default timezone is now correctly applied in TSVB if the user
changes and resets to the default the Advanced Setting timezone
parameter.

Author: markov00

src/platform/packages/shared/kbn-visualization-utils/src/get_timezone.test.ts

@@ -28,7 +28,6 @@ describe('getTimeZone', () => {
     expect(
       getTimeZone({
         get: () => 'Browser',
-        isDefault: () => true,
       } as unknown as IUiSettingsClient)
     ).toEqual('America/New_York');
   });
@@ -38,7 +37,6 @@ describe('getTimeZone', () => {
     expect(
       getTimeZone({
         get: () => timezone,
-        isDefault: () => false,
       } as unknown as IUiSettingsClient)
     ).toEqual(timezone);
   });

src/platform/packages/shared/kbn-visualization-utils/src/get_timezone.ts

@@ -14,10 +14,6 @@ import type { IUiSettingsClient } from '@kbn/core/public';
  * Get timeZone from uiSettings
  */
 export function getTimeZone(uiSettings: IUiSettingsClient) {
-  if (uiSettings.isDefault('dateFormat:tz')) {
-    const detectedTimeZone = moment.tz.guess();
-    return detectedTimeZone || moment().format('Z');
-  } else {
-    return uiSettings.get('dateFormat:tz', 'Browser');
-  }
+  const timeZone = uiSettings.get('dateFormat:tz');
+  return moment.tz.zone(timeZone)?.name ?? moment.tz.guess(true);
 }

src/platform/plugins/shared/vis_types/timeseries/public/application/components/vis_types/timeseries/vis.test.js

@@ -53,7 +53,9 @@ describe('TimeseriesVisualization', () => {
     });
 
     setUISettings({
-      get: () => ({}),
+      get: (key) => {
+        key === 'dateFormat:tz' ? 'Browser' : {};
+      },
       isDefault: () => true,
     });