Add note for Postgres 11 Support

[svelle]

As we're using cloud and also have customers already successfully running postgres 11 in production we should add a note for support

I believe Cloud uses Postgres 11 and we've been using that as well to run our load-test so I don't see any issue. If anything they should upgrade since version 9 is getting rather outdated.

Issue created from a Mattermost message by @streamer45.

[Sayanta66]

@svelle can you tell me in which document the postgres 11 note is required?

[svelle]

You could put it here for example: https://docs.mattermost.com/install/software-hardware-requirements.html#database-software

@cwarnermm what's your thought on this?

[cwarnermm]

Thanks, @svelle! That's a good place to add this information.

@Sayanta66 - I appreciate your willingness to contribute to these open documentation tickets.

[cwarnermm]

@svelle - Is action still needed on this issue?

[svelle]

@svelle - Is action still needed on this issue?

No I think since we now say 10+ this should be fine