Update first run dialog text on macOS and Linux

[rebron]

Description

Update first run dialog text on macOS and Linux to match current Windows version.

Actual result:

Expected result:

Replace "Welcome to Brave" with "Ready for the best privacy online?"
Replace "To ensure the best privacy online..." with "Set Brave as your default browser..."

Brave version (brave://version info)

Target 1.37.x/1.38.x

Version/Channel Information:

• Can you reproduce this issue with the current release? yes
• Can you reproduce this issue with the beta channel? yes
• Can you reproduce this issue with the nightly channel? yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

Related: #21694

[stephendonner]

Verified PASSED using

Brave	1.39.51 Chromium: 100.0.4896.88 (Official Build) nightly (x86_64)
Revision	4450653bfa91182e85723d8f1dee64dd6ce40ed4-refs/branch-heads/4896@{#1086}
OS	macOS Version 11.6.5 (Build 20G527)

Light theme

focused	unfocused

Dark theme

focused	unfocused

[btlechowski]

Verification passed on

Brave	1.39.100 Chromium: 101.0.4951.61 (Official Build) beta (64-bit)
Revision	3b3633b32c491b4cba5fd7df3c7c0f628547cbcc-refs/branch-heads/4951@{#1189}
OS	Ubuntu 18.04 LTS

Light mode:

Dark mode:

Polish locale: