Add support link to OS deprecation message

Per !1019 (comment 3030179)

@donuts here's what it currently looks like:

Thanks @henry! I'm assuming the button and [X] do the same thing, is that correct? If so, I'd maybe forego the "Got it" button.

I also added different wording (and icon) to be always shown on 1st October and later.

Could we stick with the same ⚠️ icon please? However the updated string is great.

Here's a slight tweak to the wording:

• { -brand-short-name } no longer supports this version of macOS. Please upgrade to macOS 10.15 to continue receiving security updates.

How would this interact with the current language/locale message bar that appears on first run?

I'm pretty sure they stack.

Also, your "Learn more" link would be do-able, but I'm not sure where it is meant to point to. Note we show this notification before connecting to Tor.

I thought Community were working on a support entry, but I was actually thinking of this issue: tpo/community/support#40136 (closed)

Nonetheless, I think given the number of users that may be impacted, a proper support entry may be warranted – however we can figure that out later.