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