Adjust CTA on desktop VPN modal

[rmcfadden3]

Description

Per this Slack thread, we'd like to adjust the CTA on the desktop VPN modal (currently, this button just says "Buy") to instead say "Start free trial" … or we could just do "Start trial" if "Start free trial" is too many characters.

CC @mattmcalister @vinny-brave @rebron @bsclifton

Steps to Reproduce

1. Open Brave
2. Click "VPN" pill in address bar
3. See "Buy" button in this modal

Expected result:

[stephendonner]

Verified PASSED using

Brave	1.53.52 Chromium: 114.0.5735.26 (Official Build) nightly (64-bit)
Revision	7075cbb66f0542ac3e01ddfde6b813e7d61118a5-refs/branch-heads/5735@{#454}
OS	Windows 10 Version 22H2 (Build 19045.2965)

Steps:

1. installed 1.53.52
2. launched Brave
3. clicked on the VPN button on the browser toolbar

Confirmed it now reads Start free trial, rather than Buy

example	example

[kjozwiak]

The above needs to be re-verified due to brave/brave-core#18524 being uplifted into 1.52.x via brave/brave-core#18573. Used #29838 (comment) as the needed verification on Nightly to uplift into 1.52.x as per brave/brave-core#18573 (review).

[kjozwiak]

The above requires 1.52.110 or higher for 1.52.x verification 👍

[stephendonner]

Verified PASSED using

Brave	1.52.112 Chromium: 114.0.5735.35 (Official Build) (64-bit)
Revision	2295354895fa3652ae47b651481831484f16d1ff-refs/branch-heads/5735@{#713}
OS	Windows 10 Version 22H2 (Build 19045.2965)

Steps:

1. installed 1.52.112
2. launched Brave
3. clicked on the VPN button in the browser toolbar

Confirmed the text change of the button label to Start free trial, from the stark Buy