Inconsistent capitalizations in main menu

[stephendonner]

Description

Inconsistent capitalizations in main menu

Note: the macOS corollary for this is #17308

Steps to Reproduce

1. new profile
2. launch Brave
3. click on the "hamburger" menu
4. look carefully at the casing of the menu items

Actual result:

Expected result:

Not 100% sure; @nullhook / @karenkliu can you help provide the correct capitalization(s)? In particular, Private Windows are referred to as a proper noun in their content page, from CTRL+Shift+N / File -> New Private window (sic). I'm also unclear on whether either string in Guest Window should be capitalized.

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.29.42 Chromium: 92.0.4515.115 (Official Build) nightly (64-bit)
Revision	48cb2f4029b84b003719740a6cf9ca73f374a857-refs/branch-heads/4515_105@{#4}
OS	Windows 10 OS Version 2009 (Build 22000.100)

/cc @rmcfadden3 @wyatt400

[stephendonner]

(This is really QA/No territory, as there aren't any Windows or Linux-specific sentence case changes, here; the changes applied to macOS over in #17308.)

Verified PASSED on Linux using

Brave	1.29.58 Chromium: 92.0.4515.131 (Official Build) beta (64-bit)
Revision	6b8d6c56ce21e38a72f7c4becb5abc1fa5134f29-refs/branch-heads/4515@{#1933}
OS	Linux

Followup issue to fix as New private window over in #17377.

---

Verified PASSED on Windows using

Brave	1.29.60 Chromium: 92.0.4515.131 (Official Build) beta (64-bit)
Revision	6b8d6c56ce21e38a72f7c4becb5abc1fa5134f29-refs/branch-heads/4515@{#1933}
OS	Windows 10 OS Version 2009 (Build 22000.132)

Followup issue to fix as New private window over in #17377.