Update Brave Talk URL in Sidebar to talk.brave.com

Description

Update Brave Talk URL in Sidebar to talk.brave.com; once https://github.com/brave/devops/issues/5168 lands, and creates an HTTP 301 Permanent Redirect from together.brave.com -> talk.brave.com, I think the following code might break Brave Talk in the Sidebar:

https://github.com/brave/brave-core/blob/ea66a8c49ade46d0d3d892626e7eea0948a95e09/components/sidebar/sidebar_service.cc#L36

https://github.com/brave/brave-core/blob/d9d0401c82d99284af60f7c81779aa632a136286/browser/ui/views/sidebar/sidebar_items_contents_view.cc#L393

Steps to Reproduce

1. after https://github.com/brave/devops/issues/5168 lands:
2. enable Sidebar via brave://flags and restart Brave
3. click on the video-camera icon in the Sidebar for Brave Talk
4. ensure it still loads and launches talk.brave.com via its Start video call button

Reproduces how often:

Unsure; this is pre-emptively filed to try to avoid/mitigate breakage on nightly.

Brave version (brave://version info)

Brave	1.28.37 Chromium: 91.0.4472.124 (Official Build) nightly (x86_64)
Revision	7345a6d1bfcaff81162a957e9b7d52649fe2ac38-refs/branch-heads/4472_114@{#6}
OS	macOS Version 11.4 (Build 20F71)

/cc @rebron @simonhong @mattmcalister @mrose17 casting the net wide; trying to get our in-product renaming done so there are no late surprises 👍

[simonhong]

@stephendonner As we reverted https://github.com/stephendonner/brave-core/commit/c323b2ea7996697dc6681424c9e82239f92719d8, this issue is still not resolved. I pushed another PR (https://github.com/brave/brave-core/pull/9349) for fixing this.

[stephendonner]

Verified FIXED using

Brave	1.29.20 Chromium: 92.0.4515.107 (Official Build) nightly (x86_64)
Revision	87a818b10553a07434ea9e2b6dccf3cbe7895134-refs/branch-heads/4515@{#1634}
OS	macOS Version 12.0 (Build 21A5284e)

(Although I made the earlier patch, it got backed out, so I feel it’s ok to verify @simonhong’s fix.)

Steps:

1. new profile
2. launched Brave
3. enabled Sidebar via brave://flags
4. relaunched
5. launched Charles Proxy and listened for traffic
6. clicked on the Brave Talk menu item in the sidebar
7. clicked on Start call

Confirmed that traffic went to talk.brave.com, as expected.