User quick range settings disappear after upgrade from 6.8.x ->7.12

[bhavyarm]

Kibana version: 6.8.14 ->7.12

Elasticsearch version: 6.8.14 ->7.12

Server OS version: darwin_x86_64

Browser version: chrome latest

Browser OS version: OS X

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

Describe the bug: If user has additional quick range settings for time picker in advanced settings in 6.8.14 and upgrades to 7.12 - Kibana doesn't display those settings (I susepct this is because of new timepicker implementation) plus there is no message warning the user that they are missing the settings in the UI.

Steps to reproduce:

1. Get 6.8.x stack up for ES/Kibana
2. Go to advanced settings and add a quick range setting - I did now to 8 years

3. upgrade to ES/Kibana 7.12 (bc1 from staging) - the user added quick timerange is not shown. And there are no messages to the user about it.

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

{"type":"log","@timestamp":"2021-02-22T12:17:17-05:00","tags":["warning","ui-settings-service"],"pid":73756,"message":"Ignore invalid UiSettings value. Error: [validation [timepicker:quickRanges].0.section]: definition for this key is missing."}
{"type":"log","@timestamp":"2021-02-22T12:17:27-05:00","tags":["warning","ui-settings-service"],"pid":73756,"message":"Ignore invalid UiSettings value. Error: [validation [timepicker:quickRanges].0.section]: definition for this key is missing."}
{"type":"log","@timestamp":"2021-02-22T12:17:37-05:00","tags":["warning","ui-settings-service"],"pid":73756,"message":"Ignore invalid UiSettings value. Error: [validation [timepicker:quickRanges].0.section]: definition for this key is missing."}

[elasticmachine]

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