(Win/Linux) New Private window should be New private window, in hamburger menu

Implementation note (from @mkarolin; updated by @bsclifton)

• The _override.grd files can’t be modified; these are auto-generated when we run npm run chromium_rebase_l10n
• The strings in the _override.grd files are based on automatic generic substitutions that are mapped in https://github.com/brave/brave-core/blob/master/script/lib/grd_string_replacements.py
• There is no mapping for Incognito -> private
• One option to solve this is to add a new string into app/brave_generated_resources.grd and then replace IDS_NEW_INCOGNITO_WINDOW with our string ID in code via a chromium_src override.

Original issue description

New Private window should be New private window, in hamburger menu

Windows and Linux desktop only;macOS is taken care of in https://github.com/brave/brave-browser/issues/17308

Steps to Reproduce

• open the hamburger menu on the browser toolbar
• look at the capitalization of the menu items

Actual result:

Reads New Private window

Expected result:

New private window

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.30.3 Chromium: 92.0.4515.131 (Official Build) nightly (64-bit)
Revision	6b8d6c56ce21e38a72f7c4becb5abc1fa5134f29-refs/branch-heads/4515@{#1933}
OS	Windows 10 OS Version 2009 (Build 22000.120)

/cc @nullhook

[bsclifton]

Thanks for the attempt @SebastianLF - I removed the good first issue tag as there is some complexity here due to the replacement done for Chromium strings by Brave. More information available in https://github.com/brave/brave-core/pull/9821#discussion_r700585827

Updated the top issue with a note

[stephendonner]

Folks, thanks for your willingness to help; @SebastianLF still has this assigned to him, so I’d like to give him first attempt at solving it; thanks!