[FieldFormatters] Human-readable duration inconsistent unit casing

[Dosant]

Kibana version: master (~7.14)

Description of the problem including expected versus actual behavior:

See Hours vs hours:

Steps to reproduce:
Use duration formatting with the human-readable format and with other options.

Additional context
Noticed while testing #100540 (review)

[elasticmachine]

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

[d1v1b]

Hi, I made a change for this issue. And I made 2 changes for i18n in duration.

1. I modified to show human-readable presice as lower cased.

2. I added some i18n sentences in duration.

3. I modified human-readable approximate to be shown with moment's i18n.

Can I send a pull request for these 3 changes?

[Dosant]

@d1v1b, go ahead 👍 will reivew