PROPOSAL: unmark MQTT and alt-svc as experimental

Hi friends!

# Background

We introduced the concept of experimental features in curl a while back to
allow us to introduce new things gradually, without having to carve the API
and behavior into stone immediately. To offer keen users the ability to try
out some features and report back before they are "graduated" into proper
curl.

This simultaneously has the downside that these features are disabled by
default so the large user populations don't see them and won't use them.

# MQTT and alt-svc

Two such features I think we've seen very little use of (I say this solely
based the low bug report frequency) are MQTT and alt-svc.

I don't think they benefit anything at all by remaining experimental and
therefore I suggest we:

1. move them off the experimental list
2. enable them by default in the build

Objections?

-- 
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