Replace "Brave Web Store" with "Web Store" in 0.58.x

[mkarolin]

Result of running npm run chromium_rebase_l10n after adding
a global replacement for 'Brave Web Store' with just 'Web Store'
in brave-browser/lib/l10nUtil.js

Fixes brave/brave-browser#2625
in conjunction with brave/brave-browser#2626

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

• Start Brave with a fresh profile,
• Navigate to brave://extensions,
• Verify that the page shows text Find extensions and themes in [Web Store]?,
• Verify that clicking on Web Store takes you to the Chrome Web Store,
• Add an extension from the store,
• Return to brave://extensions page,
• Click on Details button for any extension installed from Chrome Web Store,
• On the details page, verify that the Source shows text Web Store.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source