Kibana timepicker rejects absolute datetime input if 'dateFormat' is configured to display timezone

[piehld]

Kibana version:
7.13.2 and 7.13.3, and all earlier. Assumed in the case of 7.13.4 as well.

Elasticsearch version:
7.13.2 and 7.13.3, and all earlier. Assumed in the case of 7.13.4 as well.

Server OS version:
Ubuntu 20.04

Browser version:
Chrome 91.0.4472.164

Browser OS version:
Not relevant.

Original install method (e.g. download page, yum, from source, etc.):
apt-get

Describe the bug:
If the dateFormat setting is configured to include the timezone, then the timepicker doesn't accept user input until an explicit 'z' is typed in the field in-place of the timezone.

Advanced setting: dateFormat: MMM D, YYYY @ HH:mm:ss.SSS z

And here is a screen capture of the bug:

Steps to reproduce:

1. Set dateFormat: MMM D, YYYY @ HH:mm:ss.SSS z in Advanced Settings
2. Go to the Time picker in any dashboard, visualization, discover, etc. in Kibana and try to change the absolute end or start time by manually typing it in the field.
3. The Timepicker won't like your input until you replace the actual timezone with an explicit 'z' in place of it.

Expected behavior:
You shouldn't need to type an explicit 'z' in there.

Screenshots (if relevant):
See above.

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)