Do not open brave://welcome in guest windows

[spylogsster]

Description

as requested in #28704 (comment) we need to disable brave://welcome in guest windows

Steps to Reproduce

• Open guest window
• Navigate to brave://welcome

Actual result:

• It will be successfully opened

Expected result:

• It will not be opened

Reproduces how often:

Brave version (brave://version info)

Version/Channel Information:

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

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:

[stephendonner]

Verified PASSED using

Brave	1.51.33 Chromium: 111.0.5563.64 (Official Build) nightly (x86_64)
Revision	c710e93d5b63b7095afe8c2c17df34408078439d-refs/branch-heads/5563@{#995}
OS	macOS Version 11.7.4 (Build 20G1120)

Steps:

1. installed 1.51.33
2. launched Brave
3. clicked on Open Guest Window
4. loaded brave://welcome

Confirmed I saw This site can't be reached error message