Fixes custom Brave ad notifications should not be shown on Dev channel builds - 1.20.x

[tmancey]

Uplift of #7608
Resolves brave/brave-browser#13592

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.
• You have tested your change on Nightly.
• The PR milestones match the branch they are landing to.

After you merge:

• The associated issue milestone is set to the smallest version that the changes is landed on.

[kjozwiak]

Restarting macOS due to build failing as per https://ci.brave.com/job/pr-brave-browser-issues-13592-1.20.x-macos/1/execution/node/155/log/. However, this is a known issue that was fixed via #7599. Going to attempt one more restart and if macOS fails again due to the same failure, we'll just merge as it's a known problem and has been fixed.

05:31:23  FAILED: gen/chrome/browser/resources/settings/settings.rollup.js gen/chrome/browser/resources/settings/lazy_load.rollup.js gen/chrome/browser/resources/settings/shared.rollup.js gen/chrome/browser/resources/settings/build_manifest.json

[tmancey]

Restarting macOS due to same reason as posted by @kjozwiak

[kjozwiak]

Uplift into 1.20.x approved after deliberating with @brave/uplift-approvers. QA has verified the PR on Nightly as per #7608 (comment). As per discussions via Slack, this technically can't be checked till it lands in 1.20.x but we did make sure that the PR didn't regress Nightly.